I, too, have to say that the answer is "his son."
A = viewer, me
B = man in the picture, this man
this man's father is my father's son
this man's father = my father's son
B's father = A's father's son
A's father's son = A or A's brother
Since A doesn't have any brother, there's only A, so:
B's father = A
So, B (man in picture) is A (the viewer)'s son
A = viewer, me
B = man in the picture, this man
this man's father is my father's son
this man's father = my father's son
B's father = A's father's son
A's father's son = A or A's brother
Since A doesn't have any brother, there's only A, so:
B's father = A
So, B (man in picture) is A (the viewer)'s son