This has to be totally demoralizing for both you & your son. I am so sorry this is happening. A few ideas came to mind.
1) If the school will not allow fidget items, and even your son twiddling his thumbs is too much for them to deal with. Maybe your son could wear a bracelet with a single bead on it that he could spin. With just one bead, maybe the size of a marble, it would be silent & the physical movement would not be as obvious to the teachers. I wear a ring that I can spin when I need to.
2) Send the school printouts of some ADDitude articles about helping kids like ours succeed in the classroom. I actually gave my son’s elementary school a subscription to ADDitude in hopes the teachers would read it & learn.
3) This may take some time on your part but go through the book & tally up the positive comments vs. the negative comments. The negatives will greatly outweigh the positives. I would hope that as educators they comprenend how positive reinforcement works better than nitpicking on everything a kid isn’t doing to their expectations. Maybe pointing out how one-sided their negative comments are could help change they situation. How would they feel if they received a report everyday commenting on what they didn’t do correctly each & every day.
Shame on them.