The answer as to why he's seeing other girls is pretty simple. He's having fun. He has an opportunity to try out other girls who aren't his type and see what else is out there. I'm willing to bet pretty heavily that at least part of the reason as to why the break up happened is so that he'd have an excuse to try something like this.
It's really unfair to you. He gets to go out and enjoy himself. Then use the excuse that you were having problems in the relationship to justify it. Meanwhile, he expects you to wait around for him. So in case he's still single when he's ready for a committed relationship, you can be his back-up plan.
There's no guarantee at all that he'll still be around in the future. Even this new girl that you're calling a rebound may end up being the one he decides to stay with. I've known several people who broke up with a significant other they'd dated for years, then immediately afterwards getting together with somebody new and ended up spending the rest of their lives with the new person. Contrary to popular belief, not every rebound is always destined to fail.
Ask yourself, "If fixing the relationship is that important to him, then why isn't he with you working on it right now?" The answer is that having the opportunity to be with other women is important to him than you are. I know it's only been two weeks since you guys have broken up. But honey, it's a break-up. He's out there seeing other women. That's about as permanent as it gets. You need to get some self-respect, and let go of the idea of him being "the one."
I apologize if this post is coming off a little harsh. But I'm reading about a guy who's living the bachelor life at the moment. Meanwhile, the girl who is supposedly the love of his life, the girl who saw him as the #1 man in her life, is getting nothing from him. Yet, he's trying to leave the door open with her, so he can waltz back in years later and reclaim her if he needs to. I'm getting really mad at this guy. I think you should be too. It will help you to move on.
Please go no contact with him. If you need to write a list of all the reasons why it would be a bad idea. Then when you feel like contacting him take the list out and reread it.
Best wishes. You'll get past this with time.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy