You might just have to let this unfold @sunshine244 how it does.
You say your son has accepted the other child’s new name, that there are already a group of bullies towards the trans boy not using he/him, so as far as the trans child’s parents are concerned that is bullying.
You might just have to let this play out and your son might be perceived as one of the bullies. Or it might blow over.
You can calmly say in the parents WhatsApp chat that your son struggles with changes and is very black and white on this topic being autistic.
Society used to believe that homosexuality could be cured, we now know through brain research that attraction is fixed in a part of the brain and it’s just there regardless. In time, and it does take time because you need trans people to donate their bodies to medical research when they die and there hasn’t been enough of those yet, we may discover that there is a part of the brain that is boy/girl and sometimes a boy brain ends up in a girl body, who knows?
In life we all have to nod along with things we very much do not believe. That is just something we all have to do and that isn’t going to change for autistic individuals.
If you can teach/persuade your son over the summer holidays that nodding along, accepting a pronoun regardless of his beliefs, might be the easiest way to navigate this rather than him potentially being wound up and distracted by it.
Yes this could get very messy.
Yes your son could end up being the scapegoat if more children “side with” trans boy.
As you can see it’s a loaded topic, if your son is popular in school, people follow his lead, has a big crew around him, he might be able to carry off not using preferred pronouns. But could easily be the one on the sidelines saying she’s not a he alone.
Teenagers are fickle and nasty sometimes.
And trans boy might change their mind again, it might be because the girl was getting unnecessary attention as a girl and has decided it will be better to go through secondary school as a boy.
It’s easier to shrug and say he regardless of personal belief, this isn’t a black and white topic, applying black and white thinking doesn’t apply. There will be other autistic teens who do just accept the change of pronoun. Because that’s the point everyone is different. And in these situations playing to society norms (smile politely and play along) might be the best option.