Great great grandfather
A father and son relationship is called a paternal relationship.
Depends on your part in it. If you isn't the father to either of them, then there's no "real" relationship between them. Your exwife's son would be the "stepfathers stepson" to your current wife's son's. If you're the father to the current wife's son then the other would be "his father's stepson". If you're the father to the exwife's son but not the current, then the first son is "the son of his stepfather" to the current wife's son. If you're tha father to both then they're half brothers.
your second nephew
He is your grandson. If his father is your son, he is your grandson.
My son. "My fathers son" is myself, if I don't have brothers and sisters.Therefore, "A certain man's father is my father's son" simplifies to "A certain man's father is myself".
Akbar's father's son is Akbar (or any brothers Akbar might have.)
The father of Ashanti's son should be Ashanti.
Either: Albert's son is my husband and and father of my son, Albert is my father in law. Or: If Albert's son is me, the father of my son, then Albert is my father.
The father's son is the uncle of the son of the father's daughter.
Great uncle / Great nephew
The person in question is your son. Think about it this way: Who is "your father's son"? It's you, because you have no brothers or sisters. So, you just replace "your father's son" with "you", and the riddle now reads: "...that man's father is you." Simple.
That means that the man in the photo is his son. "My father's son" is himself (since he has no brothers and sisters); since he is the father of "that man", it follows that "that man" is his son.