Oh that poor pup! That must mean he was taken away from his mommy even earlier then.
Glad to hear you took him to the vet and he has a follow up on Thurs. Did the vet give him anything for his diarrhea/tummy upset? As far as foods, I've give whatever the vet recommended for now. You can always slowly change him over once he gets better. Just look for meat as first ingredients that are specific, like duck, beef, chicken,salmon, etc and avoid any that contain generic terms like "meat" or any by-products.