A man was standing before a painting of a boy. He says "Brothers and sisters I…

Concept: In blood-relation puzzles, a self-referential phrase such as "my father's son" or "my mother's daughter" names the speaker only when the speaker has stated they have no siblings; with siblings, more than one person could fit that phrase, but ruling out siblings collapses it to a single person, the speaker. Identifying who that phrase means is always the first step before tracing the rest of the family chain.

Application (step by step for this question):

1. The man says "Brothers and sisters I have none", so he has no siblings.

2. Because he has no siblings, "my father's son" can only refer to himself, since no other son of his father exists.

3. So "the person's father is my father's son" becomes "the person's father is the man himself."

4. The painting shows a boy, and the man is stated to be that boy's father, so the boy is the man's son.

Cross-check: Restating from the boy's side, if the man is the boy's father, then the man is exactly the person referred to by "my father's son" once siblings are ruled out, confirming there is no contradiction. If the man had even one brother, the phrase "my father's son" would be ambiguous; the explicit "no siblings" statement is what makes the deduction unique.

Result: The person in the painting is the man's son.