I am not sure what brand your feeding him now.
when my binky got older and got sick I thought it might be her dogfood and when I read what was in it...I cried thinking omg ewww
I switched her over the dick van pattens natural balance which have formulas are made especially for puppies and dogs with sensitivities to common pet food ingredients
I'll post a link for you and hope it helps a bit my new yorkie savannah seems to prefer the chicken.
Dick Van Patten's Natural Balance Allergy Formulas for Dogs