Could be another food intolerance contributing? If he's willing, I'd look into the Specific Carbohydrate Diet to see if you can use it as a framework to eliminate any potentially problematic foods. Of course, you won't really be able to tell if he doesn't adhere strictly (eats offending foods with friends).
I'd check out The Mood Cure for more info on amino acids. L-Theanine didn't work for me, but L-tyrosine and 5-HTP helped a lot.