What Can I Do To Make Him Dump Her??

Okay, so there's this guy that I like and I think that he may like me, well I hope so, no one knows that I like him, and for some reason my friend and his friend keep saying how we should go to prom together, however there's this butch girl who asked him and ofcourse being the nice guy that he is caught in an awkward stuation he said yes. I know that he doesn't want to go with her because after she asked he seemed shocked and disapointed. So what can I do? =(