oh you poor thing. I really do think you should start by talking with the teacher rather than bringing in a letter to the head. She really is the one who has the power to make things better and is more likely to do that with enthusiasm if it doesn't come from above. Of course, if she doesn't manage a change, then you must go to the head.
It also might help to give your ds shortcuts to help him with coping. If he's having a hard time with anything in particular in class, let him skip that homework once a week or even that lesson (if you can convince the teacher to do so). I know it's counterintuitive, but sometimes kids like our sons need space rather than pressure to figure something out. My ds HATED the reading books (it's a whole teaching system) they were using to teach them to read. He was having a hard time learning to read and they were sending him for extra lessons with an aide, but she too was using the same stupid books that he hated. So, I banned the books at home and declared my dislike for them too (not hard since they really were insipid) and tried him out on a whole bunch of other books instead. In the end Goosebumps did the trick (despite the teacher's insistence that it was too hard for him). Also, maybe he can help one of the teachers or the head at lunchtime for a few weeks to just give him a bit of space to regroup his courage and also for the other kids to forget about him in the role of victim. In 2nd grade my ds started helping one of the teachers clean the playground at lunchtime and did that for several weeks at the height of some bullying he was receiving. In the end when he went back to playing, the boys who had bothered him had moved onto someone else.
Anyway, best of luck with all of it. Please let us know how it all goes.