This is something we have dealt with in triplicate. OK, maybe duplicate - difficult child 1 was always hungry, would eat just about anything.
Jody, your son is diagnosed Pervasive Developmental Disorder (PDD) and has Sensory Integration Disorder (SID) issues. So please, don't fight him on food or you will make him worse. The things tat might work with other kids, including making him go without - don't do it here. It will make it worse. I speak from experience - not only of my own kids, but I helped raise my nephews and nieces. One nephew, this didn't work with. We now are discovering, now he's in his 40s, that he's Aspie.
Jody, he's 5. That is also too young to force the issue. But I have a few things that can help. You WILL cop criticism form family/friends who will say, "Force the issue," but stand your ground. They are easier to stand up to than your son. Don't engage in battle with your son unless you know you can win. Frankly, I have learned to avoid ALL battles. That way he has come to see me (us) as helper, not obstacle. Pervasive Developmental Disorder (PDD) kids have the most amazing tenacity I have ever seen. Think what a useful tool this tenacity can be, when they learn to use it to achieve positive things! So don't make them use it to refuse you.
In what I am about to describe, I will talk mostly from difficult child 3's preferences. But I had to deal with easy child 2/difficult child 2's faddishness as well. I hadn't realised just how much until she moved out of home three months ago.
OK, here is what has worked for us:
1) I made up lots of supplies of the foods he WOULD eat and froze stuff in takeaway containers. One container at a time would be in the fridge, so serves could be doled out and microwaved on demand. I did my best to make sure what he ate was as balanced as possible. I continued to cook 'normal' food for everyone else. Microwave ovens make this easy - I keep a supply of pre-cooked bolognese sauce (much the same as taco meat - what IS this? Interesting...) and it's quick and easy to feed him separately. An interesting thing about the bolognese sauce - it had to be MY recipe, preferably cooked by me. Even husband would cook it subtly differently and difficult child 3 would complain. My best friend cooks a bolognese that difficult child 1 prefers. difficult child 3 wouldn't eat it.
2) I let him eat what he was happy to eat, so he felt secure that I wouldn't try to deceive him. He HAD to trust me.
3) Once I was sure he was secure, I would still make sure he always had his "sure" foods available but would insist he have ONE taste of what else we were eating. I did find I had to make sure that what I asked him to taste did NOT have the properties of or similarities to foods I knew he disliked. (For example, difficult child 3 hates creamy textures. easy child 2/difficult child 2 hates anything NOT with creamy texture.) difficult child 3 was more willing to have a taste when he KNEW that we wouldn't trick him or cheat him and that he could eat his favourite foods afterwards. Over time he did learn to broaden his tastes.
4) NO dessert until you've eaten your dinner - he would want ice cream for dessert. We used it to encourage, not as a threat. "Come on, only a couple more mouthfuls and then you can have your ice cream."
5) For a long time we had separate meals - difficult child 3 often needed to be fed earlier because if we waiteduntil everyone was home and ready for dinner, difficult child 3 would be too tired to eat. Besides, eating with him could be too distressing. We had family meals on weekends but during the week I'd feed him his dinner alone, early. I know it's not what is advised, but it got food into him, he was nourished, we weren't fighting, and he still got his lessons in family eating and table manners on weekends.
6) This next step, I have easy child and BF1 to thank. They came with us to New Zealand last year. Part of the delight (for me) of travel is sampling the local foods. In New Zealand we were on the road for the first week, so we were mostly eating takeaway or restaurant meals. difficult child 3 had no choice, we had no frozen supplies to fall back on. We knew we could always fill him up with bread or chips, or barbecued chicken, but it was annoying to have to leave good food and then go buy more. We have our techniques - I often wouldn't order my meal until after difficult child 3 had tried what was available. Or we would doggy-bag leftovers and someone in our group would eat it for breakfast.
But BF1 said to difficult child 3, "You need to have tastes. But I don't just want you to say whether you like it or not, I want to know WHY."
difficult child 3's language delay is in the past, but a legacy of this is his difficulty sometimes in finding the right words. So this was challenging on several levels (and I don't know if your five year old could do this). Again, we did this NOT with an air of, "You must do this because I say so!" but more as you would talk to a colleague visiting from overseas to whom you are showing the delicacy of your country. Slowly we got difficult child 3 to not only more willingly taste new foods, but to explain why he liked or disliked them. Often it came down to texture; we quickly learned to not insist, if the food clearly was creamy. But we also found some interesting things about difficult child 3 - he is highly sensitive to bitter tastes. We know husband is, so it seems difficult child 3 has inherited husband's sensitive taste buds. Where we know difficult child 3 doesn't like certain foods (such as spinach) because of something in the taste (rather than the texture) we know to not even try, if a food is too similar.
Since the trip to New Zealand difficult child 3 has been much more actively willing to try new foods. He is now actively challenging himself. I was actually partway through typing this when mother in law dropped in. She has a carton of cream she bought by mistake and wants me to have it. But on my diet, I can't eat cream. However, I had a brainwave - I asked difficult child 3 to look up his Nintendo Cooking Guide (for the DS) and search for recipes using cream. This gadget is brilliant - not yet available in the US, although the recipes have been clearly written with the US market in mind (terminology, for example). difficult child 3 loves using it and since we got it he has helped us cook a number of recipes. When he cooks something, he is more willing to taste it. He also tastes it during the cooking process.
So now we have two recipes using cream but we have settled on Chicken Pot Pie. difficult child 3 is going to help me make it.
Something else that helped difficult child 3 was when he learned about nutrition and health at school. Here is where the Obsessive Compulsive Disorder (OCD) can come in handy - when he knew that healthy eating needs balanced nutrition and a variety of food, he made more effort. He eats salad vegetables (especially tomatoes). Other vegetables - he prefers them raw.
Other help - getting difficult child 3 involved in growing herbs and vegetables. Of course herbs taste awful if you eat them straight. But add them to a recipe, and it improves it. Again, the bolognese sauce recipe of mine showed him this - I got him to help me cook and showed him how I pick herbs to put in my recipe. I got him to smell each one then I tied them together to throw in the pot with red wine. difficult child 3 was horrified - until I said that was how I always cooked his food. The alcohol boils out, I told him. The herb flavours blend in and make it even better.
It all helped him understand.
A few weeks ago I had to go out for a couple of hours so I asked difficult child 3 to pick the snow peas from the garden; "you may eat as many as you want," I told him. difficult child 3 likes to eat fresh peas because he shells them first. Even snow peas. Then he eats the pods.
difficult child 3's best friend is 10 and also high-functioning autistic. He is a shocker for not eating. His mother cooks a wide variety and gets cross when he won't eat. She's coped with him by surrounding him with a lot of change and confusion (invites the neighbourhood kids in) and I think it helps, but the payback is he wants control over his diet.
What she does - it's also what I've done - we shove food in front of them when they're busy with something else. When they're distracted it's easy to say, "Just take another bite."
I teach chess at lunchtime at the local school (where difficult child 3's friend is still a student). I teach at lunchtime and I often walk past friend and say, "Just take another bite of your sandwich," and without taking his eyes off the chess board, he reaches out, picks up his sandwich and takes another bite.
Once your child is more willing to have tastes, you may find some progress.
Think about what he eats - what do these foods have in common?
I would cut out the sweets, try to replace them with a fruit equivalent. As for vegetables - does your taco sauce have tomato in it? Because then tomato is a start, raw as a salad vegetable.
Fruit - begin with strawberries, mango, banana, pineapple and freeze it. Puree it from frozen and make a frozen smoothie with it. You can also freeze this into paddle pops (icy poles or whatever you call them). No added sugar needed, if you also use a ripe banana or ripe mango.
Vegetables - let him try them raw. I've found difficult child 3 will eat carrots a lot. He raids the fridge, sometimes I go to prepare dinner and find all the carrots and tomatoes have been eaten. difficult child 3 also likes eating the bits most people throw away. I peel carrots for dinner and I now peel them into a bowl, so difficult child 3 can eat the carrot peel. I peeled some apple yesterday (to make applesauce for pork chops) and left the peel for difficult child 3. It's OK for him to eat this, as long as the food is pesticide-free. Vegetables that we grow, he's also more willing to eat. He will pick parsley from the garden and eat that.
We've come a long way, but difficult child 3 IS nearly 15. When he was 5, he was a shocker. We would visit friends, and I would bring difficult child 3's food with me. Friends had to understand, it wasn't personal. It's part of his diagnosis and he had to feel secure and in control of what he was fed, because his Sensory Integration Disorder (SID) was too distressing.
If your difficult child won't eat "pudding" it could be the texture. My difficult child 3 loves cake, but until VERY recently he wouldn't touch cake, not even his own birthday cake, if it had cream on it. When he was younger I had to break off any icing or cream and just give him the cake bit. Then he discovered sugar. Then he made a cake for school two years ago (Black Forest cake, for German class) and made himself try it - and found the cream part wasn't too bad.
It's difficult, but if your child had dietary problems (say, had diabetes) you would have to make allowances. This is not so different.
But cut out (or cut back on) the sweets. Or maybe use them as a reward, for trying something new. Even if it was only a taste. The taco meat is perhaps the best thing for him at the moment - it's got protein, iron, some fat, hopefully some tomato - it's concentrated nutrition. See if he will take a multivitamin to help, for now. I found because of the Sensory Integration Disorder (SID) I had to get my kids onto tablets and capsules from very young, they hated liquid medicine.
Good luck with this one. It is a long-drawn out problem, this won't fix itself fast if he's Pervasive Developmental Disorder (PDD) plus Sensory Integration Disorder (SID). You will need patience, tolerance and your sense of humour. Teach him to cook. He's old enough now to learn some simple recipes. Teach him to garden and grow his own. Get some hens and teach difficult child to look after them and collect the eggs. Teach him nutrition.
You shouldn't need to make 4 different meals. Prepare difficult child's in bulk ahead of time and then you only need to make one dish for everyone, and reheat difficult child 3's.
I'd better go - I've got to go make chicken pot pies with difficult child 3.
Marg