Some thoughts from a long-term non-breakfast person:
1) This is definitely Basket C. Especially since he's only 6. You have a long way to go before he has to be seen to be eating breakfast more quickly (if ever).
2) My mother used to make me eat breakfast before heading off to school. I had an hour's travel (and more) and the sight of food made me nauseous. I was also being badly bullied, I suspect there was an anxiety component in there as well, with the result that the more my mother tried to make me eat, the worse my morning nausea became until now, I still can't eat breakfast until I've been up and awake for several hours. I tell people that I need to wait for my stomach to wake up. My mother tried all sorts of things, she really did try to vary the menu so I would be more likely to be able to face food. But it was just no good. I think if she had left me alone or just made me an extra sandwich, I would have been better off. Instead, the food sat in my stomach like a lump of lead, slowly decomposing and repeating on me.
3) I had similar problems with easy child. I remembered that I had done better when I had choice available, as well as portability. My mother might make toast for me and I would eat it on the way to the bus stop. So with the kids - I packed extra food in their lunchbox that would work as a late breakfast. I noticed their pattern on weekends and found that with easy child, she wouldn't eat anything until she had been up for hours - like me.
4) Extra things to pack for a late breakfast - easy child would often be ready to eat by the time she got to school. I remembered that when I was working, I kept a box of cereal and a carton of milk at work. I would get in to work ten minutes early and eat a bowl of cereal. When the boss arrived for our early morning allocation of tasks, he'd be drinking coffee and I would be finishing my breakfast, as we all talked. It meant I was fuelled up for the morning and not immediately hungry for a snack.
So for the kids - I kept extra supplies of cooked sausages in the fridge. The kids would happily eat them cold, or I would make a sausage sandwich for a late breakfast. A cereal bar is not generally a good idea as they are loaded with sugar and fat, they're really a candy bar in disguise. They are not a good substitute for breakfast. The best breakfast depends on what sort of diet you follow, whether you pack it with carbs or protein, but there are always portable versions of either.
Dry cereal - put it in a freezer bag. Cheerios work well with this.
An extra sandwich.
Fruit - but they need more than this if you can get it into them.
A piece of cheese.
A carton of yogurt - if they aren't into sweetened flavoured yogurt, try a serve of tzaziki with a slice of bread. easy child's favourite was tzaziki & bread with Kalamata olives and pickled octopus. A bit heavy on the salt, but plenty of protein, some carbs and no other kid would steal her lunchbox.
A boiled egg.
A cold cooked sausage.
Twiggy sticks - small salami sticks.
A cold slice of pizza (I used to make pizza using a slice of bread as a base - make it the night before and let the kids grab it out of the fridge).
Vegetable sticks and cottage cheese/corn relish dip
By putting the breakfast in portable form and stocking the lunchbox, you're ensuring that your child has access to moderately healthy and tasty food for when they ARE hungry. Forcing a kid to eat when they're not ready to can lead to eating disorders later on. On weekends, observe when and what he eats for breakfast and try to fit in timing with appropriate times for him to eat at school.
And yes, let the teachers know. I have some kids in my lunchtime class whose parents have told me that the kid often forgets to eat. I've made sure that these kids are permitted to eat lunch in my class, although food in classrooms is generally forbidden. But these kids often would skip their lunch to get to my class faster. This way as I walk around the classroom and tutor, I also remind them to have another bite.
What your child brings home should tell you if he's eating enough or you're giving him too much. Kids will eat when they're hungry - I see kids who have everything eaten by recess. difficult child 3 would often not eat a thing at school (anxiety, we know now - it's hard to eat when you're constantly nauseous). He would get home from school and demolish the contents of his lunchbox, then simply keep eating until bedtime. Now he's at home, he often won't eat until mid morning, then eats a big (generally hot) lunch, snacks on fruit & vegetables through the day and eats an early meal at night.
easy child was the youngest anorexic on the block, I used to say. She stopped eating when she was just over a year old. She wouldn't eat breakfast or lunch, she only had one bottle of formula a day and then drank water. So I mixed a raw egg in with her formula. She finally got back to eating when she was about 2. She wasn't underweight, she was doing OK, and I've been told that no child will starve themselves (unless they've actually got anorexia). This is more about eating habits and food preferences, rather than control or body image issues.
For some time easy child 2/difficult child 2 was not able to eat in the mornings, but agreed she needed a breakfast. She would make a chocolate banana egg flip. Two eggs, half a pint of milk, one banana, chocolate flavouring. If we'd boiled the eggs, given her a banana and a glass of chocolate milk it would be the same breakfast. In this case, it was all in one glass and she drank it down. OK, I wasn't thrilled about the chocolate, but she's a skinny thing and it wasn't making her fat. You can do it without the chocolate anyway, especially if the banana is really ripe.
I hope you can find some solution here that you're both happy with.
Marg