Well, that's because he didn't want to be with you in the first place. Sure, he liked/likes you but that's because you were off limits and he had no responsibility to you. You all could talk and share feelings and emotions and all that jazz but at the end of the day, you were with somebody else so he could always back out.
Now you're single - and hopefully not because of him, though I'm guessing he was part of it - and you want more, but he doesn't. Sure he's falling for you, but not in a way that makes him want to be with you. This kind of thing happens all the time. Unrequited love can't happen because of external factors. Then when it can move forward, somebody backs out.
The pressure is on him now. He said all this nice stuff that you wanted and needed to hear, but now when it's time to put or shut up, he's flaking. If I were you, I wouldn't hold my breath waiting on him to come around. For him to admit he's falling for you was a bit selfish since I'll bet he has no real plan to commit to you right now. He's keeping you around without giving you the security that you need.
Sorry, holmes. But I don't know that this one is going to end well.