@WilliamG66 You need to contact school. It doesn't matter what your son thinks of this and you need to stop this. The only way to stop this is to notify school.
You almost need to shout it from the rooftops and shine a light on what this kid is doing. Email school with all the details, the snowflake etc.
I have had to do this a few times with Ds1 who is now almost 18, including him being called a pussy because at 11 he didn't play games rated 18. School will deal with it. It comes under bullying. In ye olden days things like this could only happen in school, now with social media and constant contact it can happen outside school too.
Do not contact the mother, they always defend their child, you want this to come out of the blue, no warning. Sorry but it is true.
Make sure school know that your son has tried muting them etc and tried to deal with it but it is relentless. Tell them he is worried about being alone which is why he is trying to remain in the group.
Even though school is currently online, we get a phone call every week from either pastoral or a teacher who teaches my son, they send us messages every week about mental health and pastoral being available for anything we need. That is whole school, not specific to my son's year group or anything.
Also you sound completely lovely, I promise it does get easier but these are the shit issues that come with secondary when everyone is finding their feet. It is fantastic that he can come to you and tell you what is happening. A very mature attitude to seek out the help of adults.