TLES, I know I have been on too many of your threads to count, and I have probably been one of your harshest critics. Please take my words as sincerely and genuinely as they always have been (Sorry, just felt the need to acknowledge that we've had a chequered history).
I think it is great that you are starting to get to the bottom of things from your DS perspective. I know that in the past his behaviour has pushed you all to despair and you were given dodgy misguided quackery advice that he was just trying to get a rise out of you, and that everything was within his control.
I think last night's conversation was very illumminating, and you should spend lots of energy trying to really understand it.
Imagine if your friend invited you to go shopping with them, for a girly day. You agree, and are pleased because you rarely get time together. You turn up at your meeting place, and instead of finding your friend, you find a group of girls, some you know, others you don't. You're confused. The goal posts have moved. Then your friend announces that actually, she has some errands to run, and so instead of a girly day, you'll just scoot round the shops so she can get back. Not the most massive deal, but on top of the disappointment of not having time together, it feels like a major letdown.
Your DS obviously feels that if you say you are going to the park, THAT is what you agreed. So, there are two options.
a) You either get into the habit of making clear plans which you don't deviate from, where things like sandwich shopping are included as the plan.
or
b) You prepare in advance so that you can just go to the park.
To be honest, your DS is old enough for you to make a real difference here. Listen to him, and give him some 'DS1 space' which is treasured. Even if it just means kitting his room out with some comfy 'chill out' things from IKEA or somewhere else that is cheap.
It sounds like he is getting overloaded. Perhaps it would be worth trying to encourage a friendship with one or two people at school, that can accept the real DS for who he is?