Ok, I'll try to explain it but I do better face to face. Break down the whole thing like this:
Brothers and sisters I have none--- He is an only child
My fathers son---that is him especially since hes an only child as indicated above. So we use word association:
Brothers and sisters I have none (I am an only child) but this man's father (the prisoners dad) is my father's son (me).
Hope I explained that right.