Why don't you call the vet and explain the situation? You never know, the vet may be willing to help you with him. It never hurts to ask. They may even have a way to help you win him over.
I once sat for four hours, talking softly to the young female German Shepherd someone brought to my Grandmother (I called her Nanny - much less stuffy and formal). Lady had been terribly abused and was just terrified of people. After four hours she finally crept out and put her paw on my leg. Sometimes it just takes a measure of patience.
Try sitting, not facing the dog, looking more to the side, just talking in a soft voice, with the food dish four or five feet away. Don't try to touch him; just talk to him. Do that for a few days, putting the food a little bit closer to you each day.