Would it be worth setting up nanny cams throughout your house to video document his behaviour? Perhaps if the professionals can see long term evidence of just how difficult life is rather than just a snapshot here and there, it might help your case?
Could temporary foster care be an option where he's the only child. Just to give you all a break for a couple of weeks and a chance to regroup..
Apologies if this has already been discussed.
I wish you all the best, it sounds so incredibly difficult.