read this book:-
http://www.amazon.co.uk/Complete-Guide-Food-Allergy-Intolerance/dp/0747534306...
and ... and ... and ...
change the diet. exclude all dairy & wheat & soya & all processed foods & all soft drinks. all of them. no exceptions for 1 month. if he cheats it will not work so well. when i slip up and eat the wrong thing by mistake the bad-effects start 20 hours later and last 4-5 days. thats with just for one mouthful of bread for me :-( if he slips up the experiment will not work.
but read read read as much as you can. the book recommendations on this site are very good too, click "star" under the book section.