The first step in controlling anger is learning to communicate. Learning to listen to the message behind an action is a skill that is not necessarily inborn. Role-play with your son to show him what he can do when someone gets in line in front of him or bumps into him by accident (or whatever seems to be a trigger for his anger). Show him how to talk with the other person about the situation without hitting. Even five-year-olds can learn to make "I statements": "Susie, when you bump me I feel mad. Please don't bump me anymore."
Once we teach a child his options for handling situations, he is more likely to follow through when faced with a problem. You may want to involve your son's school counselor, who could include him in a small group on anger management or give him some individual time.