I am so sorry... you did try to help him. We always wish we could have done more. I wish this too, with my mother. She is gone now. I still go through all the things in my mind and backtrack, still trying to figure out what I could have done differently. And she is dead. It's just nuts.
I hope you can somehow accept that he is beyond your control, like Kialua said above. He really is. It is his choice that he has lived, and is now dying, this way.
You are in my thoughts.