So he has only been asking these questions for the last 6 months? Before that, he didn't do this? I wonder what happened 6 months ago to precipitate this in him? Can you think of an event that might have lead him to be super insecure?

To me it sounds like he is very obsessive/compulsive about this. If he cannot stop it even after trying and trying, then it may be helpful for him to see a therapist who is experienced at working with clients who are OC.

Best of luck. I agree with Soul Quake about not getting married until this gets sorted out.