Prison Challenge

A prison warden challenges a death row inmate that he can earn his freedom if he can create a riddle that the warden cannot solve. On the day of his execution, a man comes to visit the prisioner. The prisoner asks for the warden. When the warden comes in the prisoner gives him this riddle about his visitor.

The prisoner says: Brothers and Sisters have I none, But that man's father is my father's son. Who is that man?