I suggest you sit him down and talk it out. Don't start hitting him, violence is wrong and you don't know what his limits are.

Relationships, as others have said, is not about looks. It's about personality. Is this girl even trustworthy and faithful? You are, but he's looking at her and for all you know, she might have five boyfriends or girlfriends (or both), so you never really know. You can't judge a book by its cover, don't forget that!