That is a tough one. I think it's a culmination of things. His insecurities over feeling like he's losing you and that he failed at the marrige, or wanting to calm his fears of thinking you've "been" with someone else... maybe he's also looking for an excuse to move on himself. I think any speculation on your part will just add to the confusion. If it bothers you, you need to say something to him. Ask him, "What are you looking for when you ask me if I'm seeing anyone? Do you want me back? Are you looking for an excuse to move on?" I think being honest with him cuts out alot of the "games" that As play. You may not get an honest answer back though (one of the BIG pitfalls of the disease) so be prepared for that as well.
Just keep focusing on you.