My girls have allergies that we discovered through a blood test. Red dye is one of them and many other strange things in preservatives, etc. One of my girls is allergic to dairy and she is more hyper when she has it. In fact she craves it, which I have learned the body craves what it is allergic to. Nothing they are allergic to make them "sick". I can see headaches, stomach-aches, gas, excema, and hyperactivity. My nephew who is autistic is allergic to gluten and casin and when he has it he is more wound up, less able to control his tendincies. I do believe the foods we are allergic to have an impact on our system and in different ways. It also seems those of us with other issues to begin with seem to have a harder time of dealing with it. I have read about it and know it is talked about in the medical field. I think you have to weigh the pros and cons, is it easier to let him eat, drink what he wants or deal with his behavior? It changes depending on what the day will be like. Maybe you could find more information on the internet about food allergies and behavior. You could always look under autism because it is a large issue for many of those affected.
Good luck!