Why does my ex keep asking me this?

[Nathan]

He hasn't gotten over you yet. He hates the thought of you having another boyfriend to replace him and he especially hates the thought of you kissing him, hugging him, etc. He's hurt, and he misses you. He's grown paranoid and always wants to make sure you're still single until he can get over you. When you do get another boyfriend, he's gonna feel terrible. He's just worried about that next significant other you may have.

Oh and what I suggest you to do is to have a talk with him. Tell him, that he needs to get over you. It was fun while it lasted, but it just didn't work out and the best thing for both of you is to move on. If you still do care for him, you need to do this. If you don't he'll never get over you, and he'll hurt, and he'll be a mess. I've been there and done that, and it is not fun. Just tell him to move on, but don't mean about it.