Well, good for you for telling him off. That was a great step in the right direction!

But I still advise you to reject him. Cut ties with him completely. It'll hurt like hell for a little bit, but you may wind up thanking yourself for it later. He's not married to the girl. Breaking up with her does not consist of a costly, time-consuming legal procedure like a divorce. If he truly wanted to break up with his gf, he would have done so by now. What is this "changing the subject" nonsense? He can easily change it back. The fact that he hasn't just proves that he wants to have the best of both worlds. That's unfair to you and to his gf. Being nice to everyone else does not excuse this behavior and it does not make him a "nice person" by any stretch. He's certainly not treating you and his gf nicely or with a shred of respect. He doesn't sound like serious relationship material. It might be a sign of immaturity or it could be just how he is wired. Sorry, but I don't buy the wishy-washy excuse of dragging it out to hurt her less. It really sounds like he's stringing you along.

Now if you really want to give it a shot with this guy (which most of us are advising against, but this is your life, so do what you feel is best!)You're giving him crumbs by chatting with him online. If you really want this guy, you need to go no contact cold turkey and stick to it, make it clear that you will not speak to him or see him until he's out of the relationship. Right now, he's not taking you seriously. He's likely never going to break it off with this girl if he sees that he can keep you on the fringe like this. It's easier on him. You will make yourself seem more desirable and respectable if you do this. It also will allow you more distance and perspective.

Good luck!

I agree with Flooded though. You deserve much better.