You don't. You simply talk to him, and hang out with him. You should never openly try to come between two people, it isn't yours or anyone else's place. If he begins to like you from being around you, and seeing the kind of person you are than so be it, but if you break them up you must come to terms with the fact that if you could come between them that easily, then it won't take much for the next girl to come along and do it as well. So if he favors you over her on a level above simple eye candy, and sex appeal, then you have a better relationship. This does; however, sound like a simple crush, and maybe you should just let it go. There will be other guys out there.