If people make a comment about the muzzle you don't have to tell them what happened. You can just say that you don't want your dog to kill small animals and people might think mice or rabbits.
If you do decide on a muzzle, make sure you get a basket muzzle that allows panting. The canvas snout muzzles will keep his mouth closed.
It sounds like you doing OK with him.
Deja, GSD, sable WL, female, born 11 / 2013
Cam, Rough Collie, sable, male, born 8/2015