wookie,
There are several sites and yahoo groups that deal with positive reinforcement and aggressive behaviors in dogs. I really think that positive training will help the both of you bond and help establish his trust in you. It's a sad story to say the least. But one that sounds very familiar to me. My parents had a Springer that just out of the blue snapped when he was about six (my father will tell you that his brain continued to grow causing him pain and to therefore go crazy, but he also thinks that doberman pinschers got their name from the same thing). Patches was neutered too - just goes to show that although I do think that neutering helps with certain behavioral problems, I know it's not a cure-all. Sadly my parents did not want to work with him and turned him into a local no-kill shelter. He was adopted out - even though the staff knew of his "problems" and bit a kid. His new owner then took him out and shot him. Even though this happened 11 years ago, it's still a bone of contention between my parents and I.
Kudos to you for seeking advice and help and for being willing to work your buddy through the problems! Keep us updated on his progress, I'm sure you'll make lots of it
Did your vet suggest any medication you could put him on while you work on this? I know that there are certain meds that are like Prozac for dogs... Maybe to take the edge off his reactions until you can work through it?
Almost forgot <g>. Was that the first time he had a muzzle on? You might want to start getting him used to it gradually. You want him to associate the muzzle with good things (like cheese and tiny hotdog pieces), not the vet
The muzzle could have very well increased his anxiety while at the vet.