Kat said:
Thankfully you found out now instead of being led on for a much longer amount of time. Accept him for how he is, not just the parts of his personality that you enjoyed. He's a liar and most likely a cheater. He's made a fool of both you and his girlfriend. Ignore him and don't have any more contact. I wouldn't even be friends with someone who is so manipulative.
while I agree with Kat, I also hear you that it stinks to feel like you were special, then realize that he's been dishonest with you. The truth of the matter, though, is that his affection for you - which may very well have been genuine in its own way - probably had more to do with his ex/girlfriend and what he was missing from her than it did with you. Whether we want to judge him or not, don't lose focus of that - getting involved with someone who's already involved with someone else, or still hung up on someone else, will very rarely do you any favors.
think, instead, about finding someone who is focused only on you and isn't looking to replace someone else. when you meet someone who's really into you and not just jumping from person to person you'll notice a big difference, and it will be light-years better. Don't let yourself miss out on something like that so you can follow around someone who's already following around someone else.
you deserve better!