Witz, to be fair, there is a very good reason for you to have been so thin. The young girl I mentioned in my post above has the same condition - she was diagnosed when she was 3, but the parents were told (maybe erroneously?) that they HAD to make her eat because she would never put on more muscle mass very easily, and if she was tiring too easily so she was not eating because she was too tired - they were told they would lose her, they would be lucky if she lived to reach her teens. She's now almost 21 and studying overseas, living independently and doing brilliantly. HOWEVER - I remember when she was very young, what a struggle is was.
I also remember how much criticism she has got, and her parents have had, because she "looks anorexic". People would say she had no sense of humour or was a sad little girl, because she never smiled. I reckon she'd be a blast at poker, although she does smile now. Just not obviously; you have to know her. And any comments she gets about being anorexic - this girl can really take care of herself. Nobody says it to her twice!
But easy child 2/difficult child 2 and difficult child 3 have been REALLY thin for no sound medical reason (not just the stims - easy child 2/difficult child 2 didn't start stims until she was 10, she was already skinny before that), we've been getting the "don't you feed your child" COMMENTS, not just looks. But when this little girl stood next to my little girl - easy child 2/difficult child 2 seemed chubby by comparison.
WYS, your son is Aspie. If he's on medications, I presume it's stims? Something like Concerta? And chances are, he's got Sensory Integration Disorder (SID) in there as well, compounding the problem of what he will eat and what he won't. You also need to be careful about Obsessive Compulsive Disorder (OCD), but from what you say I don't think he is likely to obsess about "a need to lose weight". It sounds to me, from what you have said, that he is aware that he needs to eat healthy food, and that he needs to gain weight in a healthy way. He IS on your side in this (on HIS side, actually).
What really got difficult child 3 into reading nutrition labels - he was on the Elimination Diet and was highly motivated to get results. We would shop with the little booklets the hospital clinic had given us and it didn't take long before difficult child 3 had them memorised.
What we've also done - we DO give way to his Sensory Integration Disorder (SID) and cook to take his food preferences into account. We've done the same with the others. (Mind you, we're happy now easy child 2/difficult child 2 has moved out because it has made it much easier to cook more variety). But we do test him every so often: "We know you used to hate gravy on your meat, but have another small taste; your tastes may have changed."
difficult child 3 dislikes certain textures. Mostly, he hates creamy textures. This means he won't eat anything with cream in it, which means most cakes. He has come round to like butter cream frosting on cakes, but it took us making a cake and him tasting all along the way, to realise he liked it. He won't eat bananas or anything with banana, which means no smoothies. He WILL now eat my chocolate banana cake though. I swamp the banana flavour with cocoa.
easy child 2/difficult child 2 won't eat anything with "bits" in it. She WILL eat anything with creamy textures; in fact, it's about all she WILL eat. I could make chocolate cake or brownies, but I'm not permitted to put nuts in anything. Her sole exception is I can put cashews, fried, in stir-fry dishes. A lot of the time I'd go to the trouble of making a meal that difficult child 3 would eat and another that easy child 2/difficult child 2 would eat, only to find she was off her food that night. Very annoying. However, by cooking some base ingredients in bulk, it became easier to customise meals with minimal extra effort. So a big vat of bolognese sauce can be difficult child 3's nachos (corn chips, bolognese sauce, grated cheddar, all put under the grill) or easy child 2/difficult child 2's tacos (smothered in sour cream).
We have a diagram of the food pyramid inside our pantry door. When they were studying nutrition at school, difficult child 3 already knew it all and actually gave a talk to the class on how to eat healthy meals.
An Aspie kid generally gets delivered with obsessions almost built-in, as standard fitting. If you use those obsessions for good rather than evil, you can turn it to your advantage.
difficult child 3 is older than your son, we're further along the road here. Because we insist that he at least taste a different food before rejecting it, his tastes are broader than they could be. His autistic best friend (same age as your son) is a real worry to his mother, for his not eating. I teach one day a week at the school and often see difficult child 3's friend during my lunchtime lesson - I encourage the kids to eat their lunch during the lesson, I nag them all. I'll even put their rubbish in the bin for them so they don't get distracted from what they're doing. Sometimes they get so absorbed that they forget to eat; so I remind then, and without thinking about it, they reach for their sandwich or apple and eat it.
You can do the same - if he's concentrating on a book or a computer game, put food in front of him (healthy snacks - don't just try to fatten him up, he won't thank you later in life).
I just gave difficult child 3 a light lunch - we'd had a small steak left over from last night's dinner, so I reheated it for him and served it with some fresh salad.
He wanted more to eat. So I made a small batch of Chicken Supreme (yes, it's a creamy sauce - it's based on a white sauce made with stock and powdered milk, with vegetables and chicken meat in it) and I served up what turned out to be a full meal's worth. He ate the lot. An hour later he's come back asking for a snack. I told him to make himself some nachos, I had some bolognese sauce in the fridge.
I just heard him put his dishes in the dishwasher. So for lunch - he's had steak & salad; chicken supreme and rice; nachos. He'll probably want another steak for dinner, with another salad.
This has been very unusual, but when he is hungry and wants a snack, he gets s sound, balanced meal. If it means he's not hungry at mealtime - no worries, he's already HAD a meal!
I think difficult child 3 is doing this because he's on a growth spurt.
I feed my kids when they're hungry, so if they're too tired to eat later on, it's no big deal. We keep healthy food available for this. I need to cook more rice - difficult child 3 ate the last of it - but we still have a curried beef to finish, plus the bolognese sauce, plus steaks than can be quickly fried. Oh, and some chicken stock and cooked chicken for a number of possible recipes in a flash. Raw salad vegetables; raw carrots (which I either hand out whole, Bugs Bunny style, or slice into lattices and leave in a bowl like crisps).
While I love serving healthy smoothies for a fast breakfast (I used to put a raw egg in as well) some kids especially some Aspies find the texture a problem. But there are other ways! The egg can be hard-boiled and served with a dip, cold.
Good luck with this one!
Marg