I'm sorry, but that’s ridiculous.
He's leading you on. Any decent man who has any sort of respect for any women he's involved with would stick to just that woman. He's not sticking to either of you and can you guarantee he would stick to just you even if you were his gf? Probably not.
What you should do is get away from this guy. You're in college. Perfect place to meet decent guys, and this guy is certainly NOT one of them.
I'm not trying to be harsh, I had a friend in the same situation and it tore her up. She got some sense and finally left the guy and is much better off. The best advice you can get is to just move on from this guy, he's not worth your time.