Great that you got answers from the vet and he's on the road to recovery. After you get a hold of the diarrhea, I would start the transition to a better food. You can consider Royal Canin as junk food, it's good now but in the end the trouble of getting sick is not worth it. Vets get very limited education on food and usually push what they sell, so it's usually up to us owners to figure out what food is best for our fur-kids.
Oh and welcome new bully parent! Diesel is too awesome for words!