I think first you do a lot of active listening, to try to determine exactly what is going on. Just listen at first: don’t judge or give advice. Find out how your child is understanding the situation.
Perhaps your child has a lot of anger. Try to see the cause of it. School failure? Family problems? A good therapist may be a big help here.
Perhaps it is a cognitive issue. Your child simply may not understand the suffering he is causing. Then you have to try to arrange meetings with the victim. Get the victim talking. See if you can get passed the bullying incidents and nurture a friendship between your child and the victim. I’ve watched some friend go from hating the idea of gay marriage to supporting it. What happened? They got some gay friends.
If the child is small, you might read the Gesell Institute studies. Their observation was that behavior often fell apart when a child was hungry, tired, or in a social situation that was too complicated. So look at those issues as well. Never overlook a simple solution.
Most importantly, let your child know he is still loved, and you believe he is still a good person. Be gentle with him. Model good behavior. I had many high school students who wrote papers about being bullies in elementary school, and later regretting it. Bullies do grow up and change.