When your son comes home upset, I recommend that you just listen to the outpouring of his pent-up emotions. Do not try to extract details or criticize how the situation was handled. Once the outpouring stops, and your son seems calm and in control, review the incident. Ask for details and try to identify positive things your son did to ignore or avoid a confrontation. Ask him what he wants to do about the incident, stressing that so long as physical harm is not involved, he can call the shots. If the incident results in physical harm, you must report it.
Ask him how he wants you to support his plan. Even though it's hard to let him try a strategy different from one you'd choose, it's important for him to recognize that you have confidence that he can find a way to improve situations.
Encourage your son to alert school personnel to the problem. Discipline may not always be the answer. If your school has a mediation program or a conflict resolution program, ask your son to find out how they operate and consider seeking assistance. These programs generally have greater peer acceptance over tattling or getting parents to intervene.
I commend the relationship you apparently have with your son. He has open communication and support. Let him join with you in the problem-solving and you both will feel more in control of the situation.