ok wait ..
i'd like to clear..are you referring to the MAN's father or the man's FATHER?
anyway i think i answered both...
supposing i am a woman: the only person possible to be my dad's son without being my brother or sister is my HUSBAND (my father's son) and for the line 'this man's father", of course it will automatically mean that "this man" is his son, thus becomes my son, too!
now it's clear...hehe!
by the way i enjoyed the riddle thanks!