Pointing to a man in the photograph a woman said, "He is the son of m…

Let's analyze the statement step by step.

"He is the son of my paternal grandmother's only son."

1. My paternal grandmother = My father's mother.
2. My paternal grandmother's only son = My father.
3. The son of my father = My brother (or myself if the speaker is male).

Since the speaker is a woman, the man in the photograph cannot be herself. Therefore, he is her brother.