All the best to your son with having this done. I had to have mine done when I was in my mid 20's and it was a pain.

It'll be harder if he has a lot of facial or phonic tics that use the muscles around the jaw. Oral surgeons here will prescribe a fairly strong painkiller for a couple of days. This might have the added benefit of calming any potential tics. The good thing about the mouth is that it heals so fast. Great advice from everyone else. He'll feel much better generally when this is out of the way.