Hey -
I'm speaking here as a teenager with depression. I'm 17, and was bullied to different degrees all the way through my secondary school. Not to scare you, but just so you know I understand, I was thrown down stairs, had knives pulled on me and had my hair set on fire at the worst, and teased much the same as you have been describing for your son at a lesser degree. That's not to say its not distressing - I understand exactly how he must be feeling, and its bloody horrible.
I'll be honest. It took me hitting someone back to get them to leave off. I'm not a violent person, and I struggled with what i did for a while, but when people realised I wasn't going to take any more BS, they gave me some more space. I was still bullied, but no where near as badly as before.
Is there a school counciller? It helped me just having someone to go and rant at for an hour a week. If there isn't, I'd suggest looking for a local YPC or something similar.
I also second the recommendation above to seek CAMHS intervention, but bare in mind that they will probably take a few weeks to give you an initial appointment. Once you're in the system, its great, its just getting in thats the bugger.
Get in touch with the school, even if he has asked you not to. Make them aware of this individual. It might not just be your son who is being affected.
Lastly, just be there for him. Don't pressure him to talk about things if he doesn't want to - always made me feel worse - but do reassure him that you still love him, and that if he does want to talk you're here. If there is anything I can do to help, please don't hesitate to let me know.