I used to have the same issue, especially with school lunch food. But I don't have any food allergies. Id feel so sick every day at school lunch. I know all fast food stuff gives me the same terrible tummy ache. Eating a "normal sized" meal that fills me sufficiently will make me sick, I have to snack throughout the day and only ever eat one slightly less than normal sized meal a day. In a day ill have a bowl of cereal, then later on maybe a snack like fruit or a pop tart. I'll snack like that until a little past dinner time ill have whatever my fiancé is eating, but again, in moderation. I know that two slices of pizza is what will make me feel fully full- but in a short time I'd need to go lie down. So I'll only eat one. I cannot eat a "full balanced meal" because its too much, if I'm being served salad and dinner I can only choose one of them. Flavors need to cause a different sensation, the same exact taste for more than three bites will make me feel a little sick. If I need to, I'll use three different dips just so my food tastes a little different each bite. Oh and also having a flavorful drink helps when it's impossible for me to have multiple flavors on my plate.

I dunno if any of this is applicable to your son, but I sure hope you can figure out what it is sooner than later. Good luck