Your story is heartbreaking.
He sounds so trapped and in the darkest and deepest of severe depression. Unable to help himself at this stage. You must feel so desperate. I would.
All i can think is that you might contact AA, or go to an open meeting of AA and ask if anyone there might talk with you after the meeting about your situation. This might bring you comfort and information. There are meetings everywhere, every night and day.
I'm so sorry. In AA they say "Don't give up five minutes before the miracle." I pray one may happen for your brother. I pray for him to find peace.