My 11 mth. Dobie was neutered 4 days ago and won't eat anything. Vet says he's healthy & to keep trying. HELP!?

I took him to the vet. They ran all kinds of exams on him. The bill came out to be $500 and they said he's perfectly healthy and to try feeding him canned food, which does not work either. I don't know what to do anymore and my dog looks so sad. He won't eat anything at all and is the type of dog that will eat anything. Help please!!