Pinklady
Been thinking about this over the night. Am sorry if most of our responses have been outraged calls of "get the school to deal with it" - which I think should be one of your first ports of call, but is also fairly facile advice, as your own emotions must be quite complicated.
I would imagine you are feeling not only angry, but a bit guilty about not realising how long the bullying has continued, as well as frustrated by your own powerless to protect your DS.
It is incredibly hard for a school to deal with bullying (not that I am excusing the school here - I think they are not fulfilling their responsibility here), as often "wading in" can exacerbate the situation, and also it takes a lot of work to get a kid expelled. Although it can be done. (There are financial penalties for school when they exclude children though, which can make some heads quite reticient to do so).
I spoke to DH last night about this, He was bullied (physically and name-calling) for a similar amount of time in his early teens. I asked him if he felt that being kept at home would have helped. Clearly whilst not helping his long term education or social development, he did say that his overwhelming feelings were those of depression and anxiety. As he says, you have to go to school, but if you are being bullied, you don't want to go. And, as a victim, you can feel really trapped by the system. He says he was probably picked on because he was quite skinny and sickly as a kid, and lacked self esteem, so was probably a magnet for bullies (who only pick on the weakest ones in the pack, little sh*ts).
The one thing that got him through (apart from quite a bit of truancy, unfortunately), was that he joined an out of school club (similar to sea cadets) where none of the bullies went, and he was able to make new friendships and learn new skills (e.g. sailing) that built up his confidence. You wouldn't know now that he had been through such a horrible time (unless you knew him very well), and he is a pretty nice and successful guy, so I am sure your DS will get through this with the right support.
Perhaps you could think of ways to help him build up physical and mental strength outside of school. DH says having a close bond and spending time with his parents also helped him through it.
Hope this helps.