Since you’re already on title, it definitely wouldn’t be a purchase. Sounds like your plan is to give him the money for him to pay off the mortgage. I am not a tax planner, but would suggest speaking with one prior to doing this as you transferring the money to him could be considered a gift. I would think that if you directly paid off the mortgage, it would not constitute a gift, since your son never took control of the money. Sure the lender doesn’t care who pays them as long as the check clears! Once the loan is paid off, you can get title corrected with a quit claim deed that you, and your son both sign, conveying his interest in the property to you solely. Hope this helps!