john's father is the only son of my father. What is John's relationship to me?

Here's the logic, one step at a time.

"John's father is the only son of my father."

Break that down from the inside out. "The only son of my father" is you. There is only one son of your father, and that person is you. So the sentence is really saying: "John's father is me."

If you are John's father, then John is your son.

The classic trick

This riddle is designed to make you think about a third person, some unnamed man who is both your father's son and John's father. You start mapping out a family tree with extra branches, and the answer hides in plain sight: the only son of your father is you, not someone else.

Same puzzle, rephrased plainly: "John's father is me. Who is John to me?" The answer is obvious the moment you strip the indirection away.

John is your son.