Have you checked out the websites on bullying ? Have a look at http//stopbullyingnow.hrsa.gov and www.stopbullyingnow.com
Insist that the principal reads all info there and learn how he can help both the victim and the bully. This is SO important today.
There are schools that have set up anti-bullying programs where the kids are taught how to deal with bullying and how to intervene and prevent it. This is the way to go about it ! Maybe when all this has settled down a bit you can take the initiative to help set up such a program in your kids' school.
Good luck to you and your son ! By all the actions you have been taken he will at least feel reassured.