I'm confused beyond repair now. Whose child is he? The women on the boat? Then why does he have those amazing powers? It's good to have a protagonist change though, that moral loving gentleman was being repetitive somehow
Joseph is the son of the child that Erina bore. She is pregnant at the time and gave birth to Joseph's father which makes Joseph a true Joestar because Joseph's father is Erina and Jonathan Joestar's child.