OP. YABU. Hear me out...
I'm the mother of a Yr 8 boy with additional needs, and honestly, as you should know the struggles we face, having also raised a non neurotypical child for over a decade, I have to say, you are coming across very blinkered for someone with so much experience.
Our children are many things. Often super bright. Unique. Inspiring. Creative. Fabulously weird. The John Cleeses and Stephen Fry's of the world.
But, (and you know this) they can also be too in your face. Socially backwards. Honestly, sometimes just relentlessly irritating. You know this as much as I do.
No, it's not their fault. But it's not the fault of the other children either. Sorry OP, but it sounds like they just don't like him. They're not swearing, or being nasty. And yes, it's sad when it's your (my) child who's not being accepted. But this ain't the first rodeo, and it ain't the last.
This isn't a classroom lesson, where the children sit where they're told, and it seems one of the boys happily chats to your DS in that context. This is the children's social time at lunch. And they have spent a few lunchtimes with your son, and decided they don't want to spend anymore. He doesn't get to impose himself on whoever he likes and they have to accept him, or its bullying. They said no. That's something he needs to learn is ok to hear. They're not stopping him having lunch, or infact sitting somewhere else. They're allowed to eat their lunch with their friends, and you can't force your DS into that dynamic if he doesn't fit.
The school are right, and deep down, you know they are. Because that's how life works. Some people will like you, some people won't. Teaching him that the other children are wrong or unkind because he can't involve himself as he prefers in their lunch, when they simply don't want him too, is setting him up to fail.
Talk to him. Try and help him understand that there are many other places he could sit, and other children he can get to know. And some will like him. Some won't. 