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 fixed deduction allowed to every taxpayer, except those who may be claimed as a dependent by another person. Extra exemption deductions are allowed for a spouse on a joint return and for each qualifying dependent. A deduction of $3,400 is allowed for each exemption claimed on 2007 returns, but the deduction is phased out for certain high income individuals.