My female is terrified of my vet
First dog I've ever had that was..She's fine in the waiting room, fine with the techs, the vet walks in, all bets are off..what's weird is, my vet is a friend of mine, if she comes to my house Masi is fine, but I can't have her treating her at my house should something come up.
Anyhow, I muzzle her when we go to the vets, I would NEVER put my dog in a gas chamber to knock them out.
If you muzzle him, the vet SHOULD be able to jab him with a sedative to knock him out to do whatever they have to do. If they are afraid of him, maybe they could show YOU how to inject him to knock him out.