What is significant about the fact that Sohrab is the one who stops Assef? How is this accomplished?

The irony lies in the son (Sohrab) finally settling the score with Assef over the assault of his father so many years ago. Sohrab was probably unaware of Assef's crimes but he still shoots Assef with a slingshot hitting Assef's eye. The scene is very "David and Goliath". Amir was their to rescue Sohrab but, like his father, it is finally Sohrab that does the rescuing.