it doesn't really sound like allergy/intolerance to anything in particular (food issues are pretty rare really). still possible the food doesn't agree with him even if it's not an allergy.
having said that, i'd be hesitant to chagne foods when still using medication. he may have an irritated system just due to having diarhea so much so new food could make things worse - it's a toss up.
I'd talk to the vet about anything you can do to help the dog's gutt (supplements that are safe or something). something like probtics maybe.