Your father's brother's son is your first cousin.
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.
He is your cousin
An intoxicated man at a party
The speaker in "My Papa's Waltz" is the fathers son.
me (I am a man with a mother, but no brothers or sisters)
cousin!
That would be your Cousin.
Your father's brother's son is your first cousin and your daughter's second cousin.
God
Your first cousin.
Cousins
aunt
granddaughter
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".
cousin
Your cousin.
jesus