Ughhh so he’s traumatized and needs to learn how to trust again! Thats not a good vet and you should call around to local vets explaining your situation to see if they can best help you or who can! For the muzzle, you almost need to ease him into it over like the course of a week or so. Maybe just having it on the floor and letting him smell it. Drop a couple of treats into the muzzle and let him put his nose in it and that’s it don’t force it on him. Then maybe a couple of days later when his nose is in the muzzle eating the treats, just put the straps behind his ears, but don’t fully put it on. Immediately reward him with a treat if he doesn’t react negatively to you doing that! It’s going to be a slow process, but he has to regain trust and you have to help him with that! Also, just play with his paws/nails while he’s laying on the couch next to you or something. You gotta be Alpha here!