There are a couple reasons why I think their relationship changed. First, I think that Amir didn't want his friend to get hurt anymore. He knew that his friend was very loyal, and he would stand up for him no matter what (even if he gets raped). So when Amir figured this out, he decided to turn against his friend to protect him.
However, I also feel that he listened to what the older boys were saying. They said they were only friends because he worked for Amir's father, and that he was just a dog to entertain Amir. So Amir tested him out by throwing the fruit and his friend, but he wouldn't throw it back.
