S.P.
Ths site may help you: http://www.pcrm.org/health/Info_on_Veg_Diets/veg_diets_fo...
...and this one:
www.garynull.com
There could be more things that he's allergic to. There may be something missing in his diet. Sounds like he either needs more fiber, or maybe he's allergic to something else. You need to eliminate one food at a time to see.
Here is a spot on Gary Nulls site, that has articles on allergies. http://www.gnhealth.com/Documents/Topics/Allergy.htm
Good Luck.