While Baba worries that Amir does not stand up for himself, the true problem is that Amir cares too much about himself and does not stand up for others (i.e., Hassan). Amir does struggle with Assef's intimidation, but he is so self-centered that he cannot bring himself to defend Hassan.
This internal conflict on Amir's part changes when he reaches the United States. He stands up for his right to choose his major when he decides upon writing instead of medicine--a decision Baba strongly dislikes. He also takes the initiative to choose his own bride and asks Baba for help after he has made the decision.
Of course, Amir's ultimate victory is when he stands up against Assef for Sohrab's rescue. At this point, he attempts to redeem himself for being a coward so many years ago.
Amir’s true test of character comes when circumstances force him to travel back to his war-torn country to rescue Sohrab. When Amir was a child, Baba always doubted Amir’s ability to stand up for himself. He communicated his fears to Rahim Khan during their numerous conversations. Amir confirmed his father’s fears when he failed to stand up to Assef when he almost assaulted him. Hassan had to step in with his slingshot, forcing Assef to stand down. Amir had another chance to confront Assef when Assef raped Hassan, but he froze in fear.
Later in the story, Rahim Khan tells Amir about Hassan’s child, Sohrab. Sohrab is an orphan trapped in war-torn Afghanistan. The situation gets worse for the boy after he ends up in Assef’s custody. Amir is forced to travel to Afghanistan to rescue Sohrab and ends up meeting Assef. Assef challenges Amir to a fight for Sohrab. Amir has no choice but to face Assef. Finally, he stands up to the demons of his past.
Baba’s worries that Amir cannot stand up for himself change when Amir is faced with the truth about his family, he is asked to go and find Sohrab, Hassan’s son, and bring him to a better place. Amir has the choice to be a coward and wallow in his guilt if he goes back to America, or he can face his past and become good again. He stands up for himself when he chooses to find Sohrab and tries his hardest to give him a better life even if that does mean sacrificing his own safety.