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.
A sale of property that allows for tax deferment if at least one payment is received after the end of the tax year in which the sale occurs. The installment method does not apply to year-end sales of publicly traded securities. Dealers may not use the installment method. Investors with very large installment balances could face a special tax.