What does it mean when he says he can't?

It means he is young. It means he really likes you but not enough to give up being single and commit. It means part of him is saying "She's a good catch I should keep her she is cool" but the other part is saying "I don't want to be tied down, I like doing what I want without having to think of another person". The can't part is him being selfish, which is fine. Just means you have to wait or move on. He likes you but not relationships. He doesn't like you enough. He may be married.