He sounds sincere, but that means little at this point. His actions over time will determine his sincerity.

You need to do whatever it takes to regain some semblance of security with him. That means snooping to confirm NC. A keylogger is a good tool in this case. It will record all of his actions on the computer so you can confirm that he doesn't have any secret email addresses or a secret FB page, etc. Google 'eblaster'. I wouldn't let him put anything on his computer by himself. You need to do that.

It probably wouldn't hurt to hide a VAR in his car to record anything said while he 'thinks' he's in it alone.

Check his belongings to make sure he doesn't have a second cell phone. Golf bags and gym bags are typical hiding places for those, as well as the trunk or glove compartment of his car.

If he is truly repentent and there is NC, these things will show you nothing. It's worth it for you, though, to be very clear on that.

Are you working the MB plan to recover your M?