I once got slapped on the ass by a co-worker. I found it unconfortable, but I did nothing. I'd been working with her a while, it only happened once and it seemed more like an humoristic attemt than anything sexist. So I just ignored it.
Wonder how I've reacted if she was some evil biatch that actully enjoyed stuff like that and did it alot. I might have just told her to stop myself. If I couldn't I'd pretend it was no big deal at all. It's a defense mecanism (http://en.wikipedia.org/wiki/Defense...nce_Mechanisms
). If anybody asked me about it I'd most likely just say it's no big deal. It's a natural reaction. If somebody just made it stop it would be great.
If the guy is such a great guy I don't see the problem about bringing it to his attention. You don't like his behavior, be honest about it.