When Amir is finally in a position to confront Assef again, he does. It doesn't end with him winning some kind of heroic fight, Assef beats him to a pulp, but it is in this act of defiance the very decision he makes to actually take a stand that he finally finds comfort. Of course he does it to try and save Sohrab, Hassan's son, and symbolically it represents his opportunity to redeem himself from his unwillingness to stand up for his friend so long ago.
Once he has done this, he feels that he has lived up to his father's great legacy and he also learns soon thereafter that his father was harder on him because he felt guilty for not being able to admit that Hassan was also his son.
Not only did Amir not stand up for himself, he did not stand up for others either(Like Hassan when he got raped).He didn't dare to say his opinion, to the public that he and Hassan are friends because Hassan is hazara. Later that changes. He fights for Sohrab (What he really is doing is fighting back for all the times he didn't fight for Hassan) against Assef. In the fight he gets harelipped just like Hassan,I think that's a symbol that he has become as brave as Hassan. Another thing that indicates this change is that in the dreams he used to have where he couldn't part his father from the bear he later dreams of himself as the bear. He always admired his father, and his father was very brave. Bears are brave and fearless.