Poor little love. And a big hug to you. Its a horrible feeling knowing that is going on.
Ask him what he wants you to do. And if he agrees, then move him immediately.
You owe nobody an explanation, if you feel that he will have a better experience at a different school ( and he clearly will ) and he has asked, then you move him, it's as simple as that! Pay no mind to what anybody thinks or says. " This school is not right for my child" ends the conversation, surely?
If he doesnt want to move though, then as i said. Ask him what he wants you to do. There may be room to make things happen under school reasonable adjustment policies. ( You may have to remind school they exist though. They forget sometimes and double down on the easiest options for them )
Kickboxing or another martial art isnt a bad idea either.
I moved my DS ( 8) for a lot less, i was guided by him and gut instinct, and it was life changing for him. The things that have come to light since have been sickening. And to top it all off, after gaslighting me for a year, demonising him, and allowing him to be ostracized by the children of parent volunteers ( that simply had a problem with me.) .it transpired they were actually claiming for him under SEN, and was on the SEN register without my knowledge. I only found this out after i moved him.
He now receives the support he needs, has had his condition recognized, and has built up a positive social network at school. He is a different child.
Honestly just get him out of there.
Good luck. 🤗