I don't want to say that "once a cheater, always a cheater", but this is a bad sign for you. If he's carrying on a relationship with her and seeing you on the side, you have to ask yourself: what's to stop him from doing it to you if you two ever became official?

I would say this is not a man you can trust. My advice: don't wait for him to break up with this girl, just drop him. Period. Don't wait for him to reject you because it sounds like you're resigned to that fact. REJECT HIM. Embrace the inevitable pain, confide in your friends, and move on. You're still in high school? Well, you got plenty of time to find a guy who won't treat you like crap. And that is what he is doing. Stringing you along like this shows he doesn't respect you, at all.

Best of luck!