I like a guy but he has a girlfriend?

He keeps flirting with me subtly but his girlfriend and me are kind of friends and I could never do anything to hurt her. I really like this guy a lot and while thinking of her feelings I'm also thinking of mine. I'm 17 by the way. I really need help because I've liked this guy for a LONG time and I don't know what to do. We talk on the phone a lot and he acts like he likes me but I'm not sure if he does. What should I do?