Hi samalia, goteamusa, LycanMaster;
Read the statement of the question slowly and carefully..break it down into simple logical parts.
The man says, "Brothers and sisters, I have none, but that man's father, is my father's son."
1. Brothers and sisters, I have none --- means that he is the only child of his parents.
2. but that man's father --- 'that' refers to the person in the portrait.
3. (but that man's father,) is my father's son --- now my father's son means he himself, since he is an only child right?. So he is 'that man's father'...and so 'that man' is his son!
Reply if you still have any doubt..and well said bobbym.
Think about this. You would say that [Depending if you are male or female] I am my father's son/daughter. So my father's son would mean he is speaking of himself being the son of his father. See what I was saying?