OP, I'm sure you've come across people in the world of the type in about to describe. We all have.
I'm talking about nice, harmless, friendly but socially inept people. People who don't pick up on hints, subtle body language or facial expressions properly and so behave inappropriately by accident. They often seem to have few friends and always be looking for more.
If you're a nice person yourself, friendly, kind, then you tend to do things like make eye contact, smile, make small talk when they approach you. That's often all it seems to take for this type of person to decide that you're now their best friend. They start making a beeline for you every time they see you. They'll want to chat at length, completely thoughtless as to what you may be needing to do, such as get on with your work, go home, get somewhere on time or just get inside out of the rain. Oblivious to anything other than their own needs/wants, though they don't mean to be unkind and aren't necessarily being obviously rude at all in their conversation.
Hints don't work, the usual "oh I must get on now" excuses don't work, looking irritated doesn't work, giving minimal responses to discourage further conversation doesn't work, brushing them off and being less friendly doesn't work... The only way to deal with such people IME is to eventually bluntly tell them you don't have time to chat, every time. And to start avoiding them so you don't have to put up with them trying to latch onto you all the time.
This ⬆️ is what you're turning your disabled son into, OP. He's looking to you for guidance and you're royally fucking it up. You're doing all your DC a disservice by it. And I think you're now seeing the results of your behaviour, when you witness them telling him they don't want him around and blanking him as if he's not there. My guess is they're having to do it to avoid him trying to latch onto them, because you have been encouraging him to do exactly that.
If a group of people arranged to go somewhere and doesn't invite you, it's not socially acceptable to ask to tag along. If they wanted you along they'd have invited you. The thing to do is to say "sounds great, I'd love to do something like that one day, have fun" and wave them off on their night out, then arrange to do something similar with your own friends if you want. That way they know you'd be up for it in future, in case they just didn't realise and so they might choose to invite you next time if it was just an oversight. Or they might not, if it was some other reason for not inviting you. It's up to them. Most people will run a mile from neediness and desperation. Asking to tag along with people's plans is exactly that. Please stop bringing up your disadvantaged disabled son to be a social pariah OP.
As for inclusiveness, to be honest this is the way of the world. People largely aren't inclusive. They want to do whatever they want to do and if you can't keep up or cope, you simply won't be invited to partake. The siblings aren't behaving especially nastily, they're behaving the way most ordinary human beings tend to behave.
They want to go to a concert, maybe drink alcohol, let their hair down, have fun... without keeping it in the back of their mind that there's someone vulnerable attached to their group who needs looking out for (and maybe physically waiting for whilst he catches up).
If he got tired or overwhelmed part way through the night, would he be able to realise it, then arrange transport and take himself home, all without their input or help? If not, then you're expecting the siblings to fulfill a carer role. And they don't want to.
It's not remotely practical to take someone along to a concert when you already have tickets and they don't. What if they'd all waited in the (probably much longer) queue for him to purchase a ticket instead of going straight in and then it turned out it was sold out, or the only tickets available were in a different location to theirs within the venue, or were way over his budget? Would they all have been expected to go home again with him and miss out?
Concerts are expensive, they have probably saved and looked forward to this trip for some time now and they want to fully enjoy it, unencumbered by responsibility for anyone other than themselves. There's nothing wrong in that.
You have massively underestimated the impact of your parenting decisions surrounding DS's disability, upon your other DC. It doesn't matter what other experiences they had, the sheer disappointment of having plans cancelled for any reason is so so frustrating for a child and a very big deal to them at the time. Compounding that with expecting them to suck it up and not be disappointed, or express their upset at the circumstances, will have made things so much worse (and I have no doubt that your parenting style included doing exactly that).
It sounds to me like what changed was they were actually very patient and kind all their childhood and did suck it up without complaining whilst their brother was a child, but that once he turned 16 and started developing his own friends and social life they saw no reason for him to be babied any more and made center of attention within the family still. They're pushing back against your unfair and unreasonable attempts to continue doing that.