We only meet Sohrab at the very end of the novel – so there's not much room for Hosseini to develop this character. Hosseini does tell us, however, just how much Sohrab resembles Hassan. When Amir finally meets Sohrab, he says "[t]he resemblance [to Hassan] was breathtaking" (22.49). Like Hassan, Sohrab is a whiz with a slingshot. He's also fairly perceptive for someone so young. When Amir tries to explain to Sohrab why Baba didn't admit that he fathered Hassan, Sohrab catches right on: "Because [Hassan] was a Hazara?" (24.112).
Sohrab also seems to have Hassan's innate goodness. You might expect Sohrab to lash out more often at Amir, or to take some sort of revenge since Amir almost abandoned him. That's not the case. Sohrab does remain silent for a year, but it seems more like detachment from the world in general than anger at Amir. And this is the one major difference between Hassan and Sohrab. The cruelty of people like Assef defeats Sohrab. When Amir tells Sohrab he's going to put him (briefly) in an orphanage, Sohrab tries to commit suicide. We believe Sohrab is not trying to hurt Amir – he's just given up. Hassan never gets to a point where he gives in to defeat, even though he, too, is raped and betrayed.
It's possible Hosseini wants to show us, through Sohrab, that a country can only take so much brutality. The first and second generations may be able to avoid the total cynicism of suicide. But not the next generation – the third round of betrayal and abuse is too much.