Hmmmm, Bliz, I came up with the same thing as Hooked (he beat me by one minute)
If the person in question was your son, then his father is you, and his father's father is his grandad (your father), and you are your father's son.
His (your son, the person in question) Father's (you) Father (your dad) in my Father's son (you)......
Um, never mind, that don't work either.....
Damn, and I KNOW this one......