What you should do is remove lust and feelings and look at this with harsh critical eyes.
This guy has a girlfriend that he has been dating for a while, and he is giving single/available signals to you. So if you get him, what have you really gotten? Either a cheater or a guy who is eager to dump his girlfriend for the next pretty little flirty gal that comes along.
You should back off and see if he persues you. If he does you should ask him "what about your girlfriend", and if he has any other response than "I broke up with her" so that he could be available to date you then he's a creap.