Well, I would think he comes back because it's easier to keep you around while he's still searching for your replacement - you're probably like a comfort blanket to him. Once he finds someone else he wants to be with full time, then he'll probably move on himself. It really is a win-win for him and you're the one permitting him to continue.
You can either keep putting up with someone who gives you less than you want, knowing he might kick you aside at any moment. Or you can be proactive and break up now, knowing that he's just not into you anymore and that the little he's prepared to give is not enough for you.
I would suggest the latter option. It might feel bad in the short term to be assertive like that, but if you let things carry on, it'll feel a whole lot worse when he finally leaves.