How is your DS? (you rarely mention how he is feeling about this situation, which makes me wonder how he is reacting to it - possibly it is not affecting him as much as it is affecting you?).
Does he want to leave the school, because if it is just this other child, then you owe it to him to try to sort this out with the school - before you pull him out, take him away from his friends and familiar places, and put yourselves under additional stress paying for a new school (most parents who go independent plan for it as it is a lot of £££, as you must know). You need to factor in these new potential stresses before you pull him out.
It really would be dangerous to swap one source of stress (this child) for a myriad of other ones, if there is a chance of resolving this another way.
From your previous posts, it seems like you haven't been to the Head or Governers dierctly about this, despite it going on for a long time. You must do so, and give them a metaphorical kick up the bum to resolve the situation. You have to go to the top, and be direct. Not emailing them, or leaving messages, waiting to hear from the teacher etc. My advice would be to go in on Monday, sit outside the Head's office until he sees you. The only way the school can get you to leave is to call the Police
. Once tyou have the head, ask for a meeting that day or the next with the Governer responsible for Safeguarding, and then make sure you come out with a effective response. It is part of a Head's responsibilities to deal with things like this as they arise, so if you are assertive enough, you will get to see him.
Only by doing this are you giving your son the best chance to stay at the school. I am sorry to sound harsh, but to me it sounds like you have pussy-footed around this by not going to the top (irrespective of your opinion of them, it is what you should do in this situation), and are now ducking out of confronting the powers that be, but still being indignant and upset.
Take control FAB, and only if you have explored every avenue, should you take this huge step for your DS and the rest of the family. I mean this with every kindness - and it may be hard to hear, but please give what I say some thought.