itati- my situation is slightly different because my son is non-verbal and severely autistic. He rarely hits me because he's cross (he bites himself when he's cross), but he just hits me repeatedly each day because it's a way to get rid of energy I think or because he's over stimulated. It's a recent thing and I haven't quite worked it out yet. It might be to try and get a reaction. He tends to not do it to men though. DH thinks it might be a height thing as the one time he has hit dh ds1 was standing on a box so was the same height as him.
He's 9 now and I think some of it might be that wanting to wrestle thing (his 2 younger brothers wrestle) inappropriately applied.
He's also much stronger than me, so if I need to make a statement that his behaviour is absolutely unacceptable then I shove him into his room (or out the room he's in if I feel like I can't get him up the stairs) and hold the door shut until he calms down.
I sometimes have to try and contain his lashing out myself using my arms, but it's really hard. Much easier to put a door betweeen you if possible.