OP, I really feel for you. My situation isn't nearly as critical as yours, though. DH also works from home a great deal (his workplace has been pretty hot on home working during the pandemic, and he's now on sabbatical!!) and initially I found it very hard adjusting to sharing 'my' space, even though I know I know, it's his home too, etc. BUT he has a paying job with a private office of his own (though he complains he's constantly distracted by people there). He, like so many others forced to work at home, discovered the benefits of home comforts, no commute, etc, and I can't blame him.
But I think it's important to get out of the house. I think it's important to have a work zone (even though the nature of his work means he brings back research, marking etc all the time) separate from the place you relax and have as a haven. And I think it's vital to have a workplace where you interact with other people, not just via a screen. I don't do paid work any more (mostly because I burnt my boats but also because we have a severely autistic child and the hours would be tricky) and really miss these things.
However, DH isn't the selfish prick of the OP. During the worst of lockdown when we had two kids at home, one of whom definitely wouldn't settle to any sort of school work and it was mostly a matter of containing him (ASD), DH had to continue with work meetings and teaching and so on and managed to find places to do this - in the bedroom, in his car, on the patio in nice weather... Later on when pubs opened, he's been known to take his laptop down there (the pub sometimes lets him use an empty room!). He finds a space because he does understand how difficult it is for the rest of us to have to hush when he's on a Teams meeting, recording lectures etc. And while I've had my moments of 'I can't stand this any more! I need some space and peace!' on the whole it's been more of a benefit than a drawback. We have the occasional sneaky lunch out together. He's at home when DS's school transport gets back and can be flexible around that.
But I do still miss my space and time alone. And if I'm having a bit of a bleak, slumped-on-the-sofa day, I don't want my inactivity to be observed. Sometimes I worry that I've now got so used to having him at home that when he does go back, I'll find it really hard to adjust back again!
OP, your DH has what sounds like a perfectly good workspace upstairs if he's going to insist on WFH (when he doesn't need to). You have to be firm about this. He's not the only person in the house who has specific needs - your son needs his space to be himself too, not tiptoeing around. Tell him to clear his clutter out and get a better connection sorted, and read him the bloody riot act! Sounds as if he lacks imagination if he can't understand how important this is.