I don't know how you could've been with him while he was still living with his Ex. I would be in a nervous wreck if I was in a relationship like that.

. You must've totally trusted the guy with that situation to be able to let him do that.

. That would've really put a strain on you.
If I were you now I would make sure that he has cut off all contact with his Ex. No number on his mobile, home phone or anything. If you still want to be with him.
Then again if he gets away with it this time. What's the chance of him doing it to you again? If you really do want to be with him again. I would give him one more chance and if he cheats on you again leave him. Everybody deserves a second chance I guess.
Try not to get to attached to this guy and pin all of your hopes up on him. Easier said then done but. Good Luck.
It sux when a guy doesn't know what he wants or who he wants in his life. I have been there.