He had already mentioned that he is not seeing anyone. The issue is whether you believe him or not. People can say different things and not mean them.
He may have asked you if you are seeing someone else because he is your ex (at some point you decided to split) Perhaps he's wondering what may have caused that and trying to prevent it from happening again. That could be insecurity on his part.
If you are in a relationship, you should be able to express yourself and ask whatever you want without feeling anxious or confused. Don't text him that question. Let time show what is going on and his actions besides his words.