This is worse than just fighting. Usually it is caused by your child's insecurity and low self-esteem. He needs to bully and act powerful to hide the fear and self-doubt underlieing the behavior. Less common is a child who just likes to hurt people and control them. When this is what is going on, the young person is extremely disturbed.
Watch your child, his behavior, and his reactions. Focus your attention on these questions;
These questions are not the same and it does make a difference. If your young person's need is to be one-up and in control, you can probably help. Your child wants other children to respect him and like him but has picked a way that does not work. They are afraid of him but neither respect him nor like him much. Say, for example, "Bullying and picking on Susie isn't a good way to get the respect you want. It just makes her afraid of you and causes others not to like you. It doesn't get you what you want. Even those who act like they think you are a big deal aren't being honest with you. They're using you to feel important. Some of them are afraid and don't want you to treat them the way you treat Susie. You'll have to decide whether you want to be a bully or have people like you."
Your child may say he does not care whether anyone likes him. If so, go on to say, "It's your choice. I'm just saying being a bully is not your best choice if you want people to be friends with you. You have choices. If you're interested, I'll think with you about some of those choices." Be sure you do not miss the opportunity to teach but carefully resist the temptation to preach.