when rahim askes to go to kabul and bring hassan's son, sohrab, back what did amir want to do? why did he change his mind?

Rahim Khan asks Amir to go to Kabul and bring Sohrab to Pakistan. Rahim Khan mentions a Christian couple he knows who run an organization in Peshawar for children who have lost their parents. Amir initially balks at the suggestion. But Rahim Khan insists. Rahim Khan tells Amir that this is not about money and tells Amir of a conversation that he had had with Baba about Amir's not being willing or able to stand up for himself. Rahim Khan also tells Amir that Baba was Hassan's biological father. Amir understands that this is both his duty and a chance to atone for his many sins.