So sorry to hear of your very difficult situation, I don't have any personal experience of situations like yours so I have very little to offer but I wondered if what my Aunty is now doing will help.
I have a cousin in their late 60's who's suffered from their late teens, there has been so much heartache for so many family members so I have a small insight into what you are going through.
Over the years my aunt, other cousins SIL's, one was a surgeon (now deceased) but he used all his contacts to get the best help even paying for private residential treatment/ counselling etc. Same outcomes as above, OK in-patient but straight back to square one once out.
Fast forward to 2016/7 and my aunt then 96 (now 98) had to say to my cousin "I CAN"T."
I can't do the washing for you, OH, family, your grand kids when they are at yours. I can't do your garden any more and your sister who helped me has had 2 heart attacks and has early onset Alzheimers so she can't help any more. Plus a lot more "I can't do ... any more".
Not because she didn't want to but just because she is no longer able to carry her AC.
Also all the siblings are now retired, 2 are widowed, one had the nursing DH for a number of years, one was on holiday and DH drown so had all that to deal with. What seemed to happen was this cousin became resentful that their siblings needed MUM too through these difficult times.
Anyway now my aunt "Can't Do" what she did, after lots of tantrums my cousin is doing better then ever before.
I think, it's just my thoughts, that as my aunt no longer feels guilty for not doing ... because she has had to recognise that she no longer "Can Do" my cousin is slowly accepting that my aunt can not make their life as they want it to be. My aunt now visits once a week, she catches the bus, will pick bits up from the local shops on her way but no longer does the supermarket shop, she washes a few pots but that's about it.
My aunt had to accept she is still my cousin's MUM but she can not fix the life of her child and she has said as much to my cousin. She is still a MUM but can not wave a magic wand (as much as she'd like to).
What she has found is a way to carry on loving her AC but is no longer even trying to "DO" anything.
She no longer searches for treatment, she no longer begs my cousin to help themselves, she long ago gave up the talks about how much other are being hurt etc. I'm guessing you've been there too many times already with those talks :-(
She has said at her age she may lose one of her children and so nearly did when the eldest had the second heart attack so if my cousin chooses to kill themselves with this illness she has done the best she could over the years and will have to be satisfied that she and the family did all they could do.
My aunt and I spoke about how hard that conversation was but now she's had it she feels better. She had to come to terms with the reality that Love however strong for another is not enough to get another person to make the choice/commitment to engage with treatment.
So if there is any way you can find to Love but Not Do, to accept that your son has to be the one who makes the choice/commitment to treatment with out you feeling too guilty, too torn apart then that is where I would suggest you put your energy.
You need to take care of you, emotionally, practically, financially and physically. If he is going to leave his marriage where will he live? I hope not with you. You are him MUM but he needs to find HIS Grown Up.
How will he take care of his children? They are HIS children and while he is your child HE is Their Parent.
If you allow him to be your 'little boy' and care for him as you would a small child - provide and home, food, pay bills and so as you would for a 5 year old he - most likely - will accept/expect all that you would do to take care of him if he were 5. All those things you did for him when he was 5.
All that care is lovely but it Dis-Ables him and can Destroy you. So please get some help for yourself. There must be organisation who know what help you'll need to best help you to support him.
The biggest thing my aunt learned was that to Love your AC does not mean that you have to fix them. She said as she ages and other things happened she had to lear to trust that she had put enough work in as a parent when my cousin was younger (before the illness) and had provided enough information, encouragement/support throughout the illness that if her AC wanted to get well/manage the illness the skills and knowledge where there and it was nat her job to fix her child.
I don't know if it's right or helpful but the change in belief about my aunts role in my cousins life has been a positive for both of them and for the wider family. I wish you all the best and hope you find a way through this difficult situation.