Oh Fluffy, I have no similar experience or advice but I just want to say how much I feel for you and your situation. The helplessness must be killing you.
All of us who are parents worry about our kids all the time, no matter how old they are or how sensible they become because bad things can happen to any of us any time.
Sorry you can't talk to many ppl in your daily (real) life for now (for obvious reasons) and hope you will be able to find some suitable support group or professional help soon.
In the meantime, perhaps chatting on here will help you to release some of your fears and tension?
One the one hand, your DS might need to go much further down the spiral. (So you need to be semi-prepared for that.)
On the other hand, he might pass thru some sort of a crisis very soon which makes him step back from the brink. Or he might just get bored and decide to make some changes. Who can say?
Whichever way it will go, maybe there is nothing more you can really do to influence your son for now, apart from to keep on loving him from afar while protecting yourself and your other children? And, horrible as this must sound, you might have to just stand back and wait to see what your DS decides to do?
You could make yourself ill if you blame yourself and keep trying to fix him when you can't? So dump any blame you might be feeling, breathe deeply and accept whatever will be will be.
Important to create a happy atmosphere for the sake of the others for whom you must also care? (Tho clearly that's easier said than done.)
You seem to have done your best so far, so please now let the universe take over the majority of the worry, as we are all powerless sometimes and it's important for you to not be too exhausted for when your DS will (hopefully) realise that he needs your help. (Then you can spring into action.)
That's all I can think of really.
Sorry if it all sounds lame ... but I wanted to show you that I am thinking of you and hoping that everything will get much better for you very soon.