I personally wouldn't go back to that groomer.
If your animal gets injured even on accident, they should TELL YOU.
With cat grooming, you either have to be competant in yourself to handle it, or a co-worker whom you can FULLY trust to help you. Most cats are much different then grooming a dog.
If they indeed had such a hard time with your cat, and were not prepaired with the appropriate tools aka a fitting muzzle they should have contacted you and told you they will not do it. This is for the safety of themselves and your cat. This is what RESPONCIBLE groomers will do.
You should clean the wound first with a warm wash cloth or cotton ball and perhaps a mild soap, then rinse it clean. Be VERY gental, you don't really need to apply any pressure. After you can apply a very small amount of triple action anti bacterial ointments, things like neosporine or A & D. And gently massage it into the cut. Just keep your eye on him for a while make sure he doesn't lick it off emidiatly, at any rate it would not hurt him to injest that small amount.
As far as his hair growing back, that all depends on the hair folicals, if the pores were damanged they will probably not come back unfortunatly. Though I suspect once his coat grows back out, it will not be noticeable.
I might be mistaken, but I thought for the most part that cat muzzles were more of a universal sizes (one size fits all). At least from my experience.
Make sure you look him over to check for any more damage, he's probably no worse for wear, but it still didn't sound like a good experience, and I would not use this groomer again.
It's your call to decide if you want to talk to them about the cut or not. Me I personally would, ask them about it first, if they make no claim then I would continue to make them aware of what had happened, and to be sure that the owner speaks with whomever was the groomer of your cat. These type of problems will get marked on their record. Typically if one person causes so many injuries, the employer will smarten up and realize this person is not fit for the grooming business and fire them.
Mistake or not, they should have reavaluated the situation with this particular cat.