Hello OP. I hope you manage some good sleep tonight. (TLDR at the bottom if you don't have the capacity for the full comment at the moment)
I have read all your comments (I think) and many of the others from imip etc and I'm so sorry you are going through this.
But I wanted to reassure you, that there is light at the end of the tunnel because I was you 5 years ago, even down to the useless father.
My child is diagnosed PDA autistic and we've been through very similar to you, only they was uncontrollable at school, and we were lucky in that way.
I was beaten black and blue, constantly on egg shells, wondering where the next meltdown was going to come from. Not being able to 'figure out the triggers' as these change from day to day. I was too anxious to leave the house with my children.
I fought and fought and got their diagnosis, and EHCP, they were both being done concurrently, and if I remember rightly, the ehcp was done first! I know you say you have no fight, and I get it, I really do, but you must find it from somewhere. You have to be the loudest voice in the room. You are your family's advocate. You have to let them know you will not back down until you are all being supported. I even had a fight this year for their high schooling. The LEA tried to force me to apply for mainstream high. No chance. I applied for one school only. And I told them in no uncertain terms he goes where I want, or I pull him and put them through tribunals etc and he started this term with transport covered.
I know it's difficult when you have nothing left to give, but 5 years down the line you will be so thankful for exhausted you having pushed, because if you do, you will not be in this situation forever.
Now, about home.
We went NO demand. One rule only, no hitting/hurting siblings.
Anything else was fair. Didn't want to go to school, okay fine. Didn't want to go out. Okay, good. Wanted constant console access, have at it. You get the point. It's bloody difficult to go against all parenting advice and thought. I literally battled with myself that I was crazy, I was just letting them get away with anything.
But it's not that, it's about regulation. Your son is not regulated. He is in a constant state of anxiety and needs downtime.
We were in this phase for a long time, about 18 months. If not longer. It takes a long time, and genuine dedication.
Then we added school. I say we, because they are involved in all the decision-making that involves them, that they want to be inbolved in. They had to try to go to school everyday. It wasnt a difficult demand, because he enjoyed school, mostly. And that then became the norm. And so on. They now have little jobs and earn pocket money. (Still often told no though haha)
Ross Greene is a fantastic author, he wrote 'the explosive child' and it's world renowned for supporting pda-ers. I know you aren't up to reading at the moment, but he has all his work on audible as audio books. His methods work. But they aren't a quick fix, which can be frustrating, but well worth sticking with.
They now attend school, even though it's all change with this new one, we haven't had a meltdown that I can remember recently and we are going abroad for a family holiday soon. None of which seemed possible 5 years ago.
Although this approach is anti any and every parenting book going, it works for children like ours.
If that's TLDR, the main points:
Fight for his right for an EHCP. Even when you're done, when you can't go on anymore, fight.
Fight for his right to assessment and diagnosis. Even when you can't. You can.
Listen to Ross Greene and begin no/low demand parenting.
Cut yourself some slack, you are doing AMAZING. I know, I have been there. I know you're barely surviving, but fighting now, will change your lives. I guarantee it.
Sending you all my love.