As a man, I can tell you how to win his heart away from his current gal. Sever her head, bring it to him, drop it on his lap, grab his head and make him look directly in your eyes, then tell him "You are mine now." Maybe growl a bit. Trust me, we love it when women get all forceful.
Slut it up. If this is high school you're talking about it'll work, no one resists sex appeal there because as amazing as you make him sound his ultimate subconscious drive in everything he does is to get laid. And don't worry guys can keep a secret, so long as you guys are careful no one will ever know.