Gastric byp question?

My boyfriend (21) got the surgery 2 and a half years ago. He has been good since. But he gets these major stomache aches, they use to be usually after he ate something containing nuts or mayonaise. Now he had one just a little while ago and he did not have anything with either of those in it. when he gets them he whines so bad, and it hurts him so so bad. he takes perkisets or something like that, that knocks him out and makes the pain go away. any idea what could really be causing this or what else he could do to help the pain when they come again so he doesnt have to take that? thanks, Mandy