I'm not inclined to make recommendations about food without knowing more info and the info provided is a little too vague...
What are the symptoms?
Diarrhea?
Puking?
Weight loss?
How much are you feeding?
How much does he weigh?
How old is he?
Any other noteworthy medical history?
What testing has Vet done and what were the results?
If a CBC has not been done it's probably time to do it.