If you try to turn him to you, then you lose. First, you lose because he may not turn and will be offended by you trying to make him do so. Second, even if he does turn, you will ALWAYS worry that someone will grab him and you would be right to worry, after all he already was unfaithful once!
Be his friend (but just his friend) and keep looking around. You may find someone that you like better. And later he may end up liking you. Relationships change all the time.
You hope for the best and do not intervene with the current relationship he his in. If he sees that you are respectable of his decision, than he might break up with her and go to you. But, maybe not. A guy that is taken is not free game, so put your claws back in and find another prey to stalk, kitty. :P