You should ask him how many male asses he's slapping regularly without warning just to express his joy at the cameraderie and friendship. Put him in a hole there, because he sounds absolutely ridiculous.

This is specifically meant as a sexual expression and apparently it's related to other people being able to witness it in some way. It sounds like he's almost bragging for being able to do that with your butt. Perhaps ask him about that.

Maybe i'm petty, but unwanted contact makes some violence okay to use to defend. Pointy elbows, get them in action. He slaps, you poke. Just because you're dating him, doesn't mean your body stops being yours and you've already told him it's unacceptable.