I find no evidence in the text of The Kite Runner to show that Assef had any idea that Sohrab was related to Hassan. It seems that Assef would have told Amir during his lengthy talk before the fight if he had actually known. I believe it is pure coincidence that Assef chose Sohrab from the orphanage, and there is no evidence that he could have possibly known his name or his origins. Amir never mentions the possibility in his narrative; likewise, Sohrab never mentions this notion to Amir afterward. Although it would have certainly been fitting revenge for Assef--to molest and rape the son of the Hazara he hated so much--Assef apparently did not see the connection between the father and son and their prowess with the slingshots.