Step 1: Really listen to your child.
Make sure they feel they can tell you anything without fear of being punished. Ask the child if he wants both parents to listen or just you? Be a good listener and let them speak as long as needed. You might want to write down a few main points and discuss them afterward you have listened silently. Understand they are only interested in their problem at this time and not your problems in the past. Stay focused and look for any clues that suggest specific ideas on how to help. Pay special attention to signs of hopelessness which could result in actions dangerous to your child or others. Try to appear non-judgmental and confident without making it look like you can solve everything with one wave of your powerful magic wand. Try as much as possible to be open about discussing possible solutions: actions the child can take and actions you might take. One issue you should discuss is whether the child has informed the teacher. You should make it clear that a first step you must take is informing the teacher, but this will be done discretely. Make sure the child understands teachers are given special training in how to work behind the scenes in bullying situations. Make sure they know they should not try to go this alone. You are here for them and so are their teachers.
So what have I left out? What should Step 2 be?