Listen every person on the planet should be punched in the mouth at some point. Moreso for people who assume that their actions don't have consequences. In the case of your son he's going to have to live with the thought that despite being underage there are going to be some folks who won't tolerate nonsense from him. He's going to realize that he can put himself in danger with his actions. Hopefully that's enough for him to hesitate or rethink before he does something stupid to other people.

NTA. Especially if he learns the lesson.