Why does anything matter when he chooses to stay with his girlfriend? He sounds like a man with very, very little integrity. He's not breaking your heart, you're not a victim - You're choosing to get in too deep with someone who is living a complicated life, whose love life is very confusing and obscure, and who is leading probably two or three women on right now.
Perhaps he just likes flirting with you. Men who have no integrity and enjoy leading women on will never talk much about the girlfriend who is essentially being cheated on. Do you eventually want to be that girlfriend? Because guys like that don't change just for one woman they happen to be attracted to.
You're really getting into dangerous territory here and you're only going to come out hurt. If you enjoy flirting and the attention then this guy is clearly available but if you ever wanted anything more you'd be left disappointed.