You have no income tax liability, but depending upon the size of the remaining balance you may face federal gift tax issues. Your son has no tax liability; your generosity in paying off his mortgage is a gift to him.
Debt on which a person is not personally liable. In case of nonpayment, the creditor must foreclose on property securing the debt. At-risk rules generally bar losses where there is nonrecourse financing, but an exception applies to certain nonrecourse financing for real estate.