Thank you all for the advice and supportive messages.
The wide variety of responses makes me feel quite validated in not knowing what to do too, as there's been everything from "he's got you wrapped around his finger, put a lock on his door" to needing to take him to a psychologist.
I think my next step is going to be carrying on with the mattress on his floor (him sleeping on his own is way beyond the realm of possibility right now) but reinforcing the boundaries of needing to be quiet and not waking me when I am there. Also appreciate PPs saying boundaries should actually help if it is anxiety.
I'll also try a couple of the things mentioned (weighted blanket and sleep gummies - even just placebo effect might help). With the weighted blankets though, everything I read says to only use 20mins at a time/not keep them all night? If so, it might not be much use.
If all continues to fail I certainly will consider a psychologist, if it is anxiety it can only help and I'd like to tackle it sooner rather than later.
I know what I described in the night with the water and shouting makes him sound like he is naughty, but this has always been his response to anxious situations (lashing out/being horrible), I genuinely don't believe it is naughtiness. He is truly lovely the rest of the time.
Regarding good sleep hygiene, we are already doing everything. He's always had a good routine, healthy diet, plenty of exercise, barely any screen time. Never had sleep crutches until this all started. We have also been doing occasional journaling and the kids have been asking to do bedtime yoga every night which has been lovely.
I got his journal back out today and we were chatting in bed a while and decided to do a few pages. On the "worry" page he could not think what to write and just wrote "nothing, just happiness all together" and seemed to mean it!
I do also see what PP are saying about putting him in with DS2 but this would make me feel really selfish, it shouldn't be DS2's problem to resolve. He also sleeps so peacefully I'd worry he would get disturbed by DS1.
Onwards and upwards, we'll try to get through the day and go to sleep at the same time as him tonight. He doesn't usually wake for the first 3 or 4 hours but this could be because he knows we are still up and the house isn't deadly silent.
My watch says I got 2h9mins sleep last night, it is actually insane.