There is a lot of information on this on the web, just stick in food allergy or intolerance in google. I found a simplified website that explains the difference between a true allergy and an intolerance:

http://www.foodallergynews.com/Gene...Information.htm

This site also lists the most common foods people react to-that would help you start an elimination diet. I cannot eat broccoli or at least the florets-they make me dreadfully sick for about 12 hours. It took me FOREVER to figure that out. You might also want to look at MSG as a culprit. Many people are sensitive to that food additive and glutamate is found in many natural foods (broccoli among them).

http://www.msgfacts.com/lookatfacts.html

Lastly, you might want to consult your doctor or a nutritionist. They can do tests for an actual allergy or help you determine what foods he is reacting to if it is an intolerance.

Keep us posted!