Well, everyone has different preferences for this sort of thing. =P You could do one of two things:
1) BEFORE you get "hot and heavy", discuss what you may or may not like. If you like his grabbing your butt, let him know! If you don't like him spanking you, on the other hand, let him know about that as well. If you aren't sure, then you could suggest he try it out next time.
2) Be spontaneous. Pull HIM on top of YOU, and do to him what he was doing to you, or switch it up a bit. Hey, who said only GIRLS like to have their butt grabbed?!