The Wordy Puzzle

"Brothers and sisters have I none- but that man's father is my father's son"

The writer denies having any siblings, yet one interpretation of "that man's father is my father's son" may indicate that the father has a son, other than the writer, who is "that man's father"- a contradiction on first sight.

If I were to hazard an interpretation that does not introduce contradictions, I would consider the following: the writer is an only son as supported by "brothers and sisters have I none", and the writer is in fact referring to himself in the third person with "my father's son". Then, "that man" is the writer's own son.