Sounds to me like he likes you. But y'know. Don't hold me to that.
I know it's harsh, but until he does break up with the girlfriend, you cannot do a THING. If you do, you'll just get yourself a bad name and you and him may as well have just shagged right in front of her. Still just as bad.
So yeah. Sounds like he likes you, but don't do anything 'til her breaks up with her. If he doesn't... well, hate to be harsh, but move on. I know it's more difficult than that (believe me), but it's just how it is sometimes.