My dog is sick, but why?

Oh my, I know my vet would have done more than just recomend something. We've taken dogs in with upset stomaches before and he did a LOT more. Make sure he drinks water. Take him back to the vet, or to a different one. Some dog's noses are dry. If he's not acting like himself, and not wanting to eat, take him back within a day or so. You seem like a good caretaker, it's good that you worry, even though your vet thinks that it isn't anything serious. At least call your vet and tell them what's going on and ask them. Good luck!