I'm neither a parent nor a teacher, so I may be completely off base here, but I feel like this perhaps isn't something you should address. If the teacher had called your son "stupid", then I'd say it definitely needs addressing, but I think that if you write to the teacher because you're unhappy with a word he used (which, while not the best word, isn't an offensive term) you'll come off as petty and helicoptery. I'm not saying you actually are a helicopter parent, because I completely understand why you're not pleased, but I just don't think this is a hill to die on.
Make sure your son knows that you don't think he's stupid, and let him know that even though what he did was thoughtless, he apologized and it's not the end of the world. As for the slapping, I did pretty much the same thing as a child when I felt I'd done something wrong. I grew out of it when, with age, I learned to control my emotions better, and I'm sure your son will too.
My only caveat would be that if the calling things "stupid" becomes a habit with this teacher, then maybe you could consider addressing it. It doesn't seem like this is the case though, so as unpleasant as it is, I'd leave it alone.