Did the breeder ever worm this pup? When the pup went to the vet the first time, did they check his stool? At his age, my guess is the diarrhea is worms. All pups need to be wormed on a regular schedule. If you're in the US, I believe it's illegal in most states to sell a pup less than 8 weeks. It's certainly NOT advisable! The breeder should have given the first set of shots at least and at least one round of worming meds. I'd have the appt. moved up and make sure they do a fecal check for all types of worms and infections. Then you'll know if it was the food or not. Be sure to take the pup back for another stool check... which may be before he's due for more vaccines.