Ugh, what an awful situation.
It does sound like you've got a principal who is responding appropriately. DD also had a silent signal for her teacher for bullying. It was empowering for her.
Now it's time to address the bigger issues:
*Who is going to address your son's social skills so that he's less of an attractor of such behavior? It sounds like he can use some social skills training.
*How will the principal address the climate in the classroom, lunch room, and playground such that these issues keep happening? If your son was the recipient of so much such behavior, I'll bet you he's not the only one.
Evidence-based, school wide anti-bullying programs work. I would meet with the principal in a week or two as a follow up, and ask what he's doing school wide.
