No. Don't send it. Don't do anything. Less is more.
He may very well be able to start with a clean sheet, but he won't if you send this. Your view on this all is way off. Take a step back don't do anything. wait to see how it goes first.
I'm struggling to grasp this. Bullying but none of the other parents know? No, there's always a couple of kids who tell their parents everything.
The school seem to have failed to address it. They have failed you. Why haven't you asked for more to be done.
Every single child in the class participated in this 'stay away from ds' game? No. I doubt that. In a class there are always loads of different personalities : some are outgoing and vocal, a bully maybe or two, Some are very shy, some compliant, some do things without even realising it. to insinuate that they all did it is very very rare. I bet you there's loads of kids who don't even know that the game is going on. Bet you there's a group of 3 girls all friends who know nothing about it because they just play together happily every day. and there are some very gentle souls who are very kind who would never ever do this. There always one or two of those. Always. there also some children who always tell their parents everything or there's always one who Dobbs everyone into the teacher so I failed to see that all the kids in the class did this is to me it's just practically impossible. I'm not disputing that quite a few children did it to your child there was such a game but the teacher should have dealt with it and the school is failing into this should be your primary focus and re addressing it.
Why didn't they set things up to find him a new friend? to make sure he was ok. and there are loads and loads of things that schools could have done to facilitate at least one friend and none of these things seem to have been done, but you don't seem to be looking at this.
You need to take a step back because you're too focused. You can't see the bigger picture. Sending this message no good will come of it all most people just think it was very odd.
What about trying to address the issues from another angle? Speak to the new school tell them get things facilitated from the start. get him set up with new friends, get them to keep an eye on it, and do all this it in an underhand private way that no one else knows about it, only the head of year and the teacher at the school will know. Surely this will be much more proactive and productive.