Child support for child born in 1979 still attached my wages?

Your issue is with whatever court issued the garnishment of Social Security checks not with Social Security who has no choice in the matter. Have you tried to duplicate the documentation you once had proving he had been living with you? What did you have? Statements from your ex? Your son? Neighbors? Get them again. How about the school he went to? They still have the records. If you are on good terms with your son get a statement from him about what he remembers.