Method one: If your son was given title to the property or made a co-owner to obtain a mortgage, the retitling of the property interest back to you should take place. The title company will prepare a deed to retitle the property. The title company will record the deed and the mortgage satisfaction correctly.
Method two: It will be easy if your son used your home as collateral or cosigned a mortgage. Just record the satisfaction at the courthouse. The lender typically will have the mortgage satisfaction drafted, and the title company will record the satisfaction.
Is there a written agreement? The problem with oral contracts, even between family members, is that the details are sometimes remembered differently with time. The further back in time, the greater the chance of a misunderstanding. I mention this only because you could have asked your son instead of asking Monty.
If the two of you are not in agreement, the time to resolve the issue is now. You may want to contact a local mediation group to learn how they can help. My experience is that mediation is faster, more private and considerably less expensive than litigating. Here is a link to the Academy of Professional Family Mediators, a website with a directory to locate a mediator in your area. This link is not an endorsement as I have not used this site, nor would I receive compensation if you were to connect with a mediator.
If you are unsure whose name the title is in today, you can call the title company, the lender or your son, or order a title search to learn the answer. The cost of a title search varies around the U.S. In my county, the price of a title search is $75.
Once you pay off the mortgage and resolve any title issues, it will be your home again.