Feeling rejected by people is a tough thing to go through as a child. When your child complains that nobody likes him, you can help your child belong by taking a little time to teach him how to make friends, and he'll feel great in no time.
First tip - Listening
Tell him to see things from your friend's point of view.
Every child wants to be understood. Your child might have to ask his friend questions in order find out what he is thinking. He could start by saying, "Why are you so mad at your sister?" This opens a door to talk about the problem. Tell your child to listen very careful after which he might reply with a comment like, "I can now see why you're so mad."
Note here that your child might not agree with his friend's point...