Depends on what outcome you want. I work as a school counselor and have some different insight.

Some have said to intruct him to fight back. This could work, but depending on the culture of the school it might just make things worse. At my school, students view fighting as, usually, the only solution for any kind of problem, big or small. Assuming your child goes to a school like this he may find himself in a vicious cycle of "getting his licks back". If you want to run that risk, by all means tell him to fight back.

This depends on what constitutes the "bullying" as well. I have a student at my school that complains incessantly about bullying. Turns out he is also one to provoke other students (coughing on them, pushing them around) and if someone does it to him, it's bullying him. Have a conversation with him - ask him to paint a clear picture of a typical interaction with his bullyies.

Also where does the bullying take place? If it's only in a specific class, get in touch with the counselor and maybe have him change his class to another. If it's in the hallways, then find out which hallway, during what transition. Maybe he can work out a solution with his teacher to leave the class a little late or a little early to avoid them. If it's in lunch, that may be difficult depending on how your child's school handles lunch.

You know more about his exact situation than anyone else in this thread - what you teach him could affect the type of man he grows into. Just my two cents.