Why would Assef choose Sohrab over all the other kids?

Assef still despised Amir and Hassan. Assef wanted to torture both men more through the abuse of this child. Assef was truly a disgusting person.

Yes, Assef would torture Hassan even in the afterlife!

Asset hates Hazara's and any Hazara child reminded him of Hassan and his hatred of that ethnic group. Also Sohrab reminded him of Hassan because of his appearance and race, and believes that they should all be suffering until they go back to Hazarajhat.

Assef*