Most Helpful Guy
Well I don't know for sure since I don't know anything about him or the situation, other than what you've said (which is actually quite little) but I can speculate the most likely scenarios.
1: He wasn't intentionally lying and he honestly thought he was over her. Which could mean...
-- A: He thought he was over her, and found out he wasn't
-- B: He thought he was over her, and when she came back, he fell for her again
-- C: He thought he was over her, but when she came back, he found out he wasn't
2: He really was over her, but decided to date her again. Because...
-- A: Situations changed, and there's a more serious reason to get back and stay
-- B: He gave her another go, because given history, he feels he owes her that much
-- C: He was over her, but he fell for her again somehow (Not very likely)
3: He really was lying to you, and the reason for that was because...
-- A: He was putting on a front, and never got over her, but didn't wanna admit it
-- B: He was putting on a front, and never thought he'd get back with her, so he lied
-- C: He was just saying that, because he knew you liked him and thought it helped
-- D: He knew you liked him and was leading you on (intentionally or not)
-- E: He knew you liked him but didn't want to hurt your feelings
-- F: He thought he had a better chance with you than his ex, again, so he lied
-- G: He's just a liar and a dirtbag?
-- H: Any one of a million other reasons, or a combination of the above reasons
Point is, I don't know if he even was lying. Even if he was, I have no idea what the reasons he lied could be. It's absolutely impossible for me to tell with the information you gave. Even if you told me more, the odds of me accurately predicting only improves from "Impossible" to "difficult to say for sure" and so really, I'm afraid I can't give you the answers you're probably looking for.
I can imagine you're (either) angry [and/or] hurt right now, but unfortunately, the only person who could possibly answer your question for sure, is him, and even then, who knows if he will even tell you the full truth. I'm afraid this is one thing you're probably going to have to just let go of, somehow. I know it sucks, but I can't really offer you much better.