If my DH smacked me on the arse, he'd be picking his teeth out the front garden. He knows that. He knows that without me ever having had to state it. If he had done it, he would have been firmly told, with no room for confusion, that this would be what happened if he ever slapped, or smacked, or 'patted' or 'tapped' my arse again. For whatever reason, even if he meant it as some neanderthal like compliment, and for whatever reason had lost both his reason and his words.
So in this instance, if I were OP, when DH came back, and had recovered sufficiently from getting like served with like that we could have a discussion, I would be saying to him that now he knows what it feels like to be slapped, for whatever reason, and I've made it very bloody clear to him I don't like it, and if he doesn't like it, he'd better not bloody do it again. If he feels that he cannot respect me and my boundaries and keep his slappy little mitts to himself, then I'll be seriously contemplating terminating the relationship.
So I need an undertaking from him that now he understands that I don't like being slapped, for whatever reason, on any part of my anatomy, and that he will never do it again.