Just End It
Based on your description of him here and the details of the story below, not to mention the fact that he, too, is young and relatively inexperienced, one thing is clear to me: you need to muster your strength and resolve to END this relationship (certainly the intimacy, but probably also all contact with him).
Trust Your Instincts
You make some very astute observations about him and you and the other girl involved, and you should trust your intuition. I hear you saying that you haven't gotten anywhere in the relationship, and that he may have a strong need (obsession is what you said) to be with someone, anyone.
He Doesn't Respect You or the Girlfriend
It's patently obvious at this point that he doesn't respect you or his girlfriend enough to make a decision, not even to be completely honest with her, so you have to take the initiative. It's a difficult, painful step you have to take, but it'll best serve all the parties involved in the long run.
I know what it's like to have feelings for someone who's involved with someone else. It's frustrating, confusing, sometimes heartbreaking and demeaning. These feelings are only exacerbated if the "someone" has feelings for you as well.
A Decision Has to be Made
A decision has to be made, and in this case, I believe the right thing to do is not be in contact with him for some time. He'll almost certainly try to prevent such a situation, but you must resist further advances.
Whether or not his intentions are honorable, he is manipulating you and your emotions, and keeping you from being happy, on your own or with someone else.
The answer to your ultimate question, why he's bothering with you if he doesn't plan on breaking up with the other girl, is one I'm not sure anyone can answer fully. He's confused, doesn't know exactly what he wants, needs to be with someone, perhaps gets something out of the relationship with you he doesn't get from her. Any or all of these may be factors, but it is not fair to you to be strung along while he grows up and matures.
You have your own living, loving, and learning to do, and you need to be your own guide and be with someone who'll learn with and concentrate on you, not be pulled in different directions.