The most interesting thing about this situation is that you can almost see where it's going to end up. If he dumps his present girlfriend for you, how long would it be before he dumps you for someone else? If you don't respect the relationship that he has with her, then why should he respect a relationship with you?
There is nothing wrong with being friends with someone who is "attached" to someone else romantically. But as a friend you should respect the boundaries. They would either get married and live happily ever after, or, he would end up with you and you both can live happily ever after, together.
At this point, only a casual relationship with him can win him over. Maybe he is with her now because she shows him that she respects him, and he does the same for her. Breaking up a relationship is not a good thing to do.
However, you can start a friendly conversation by saying "Hi", then asking him a question about something he is interested in. This way, he is likely to talk for a long time, and will know that you are interested in something that he knows something about.