Question on untreated gangrene.?

My father has had diabetes for many years, earlier this year he developed gangrene and two of his toes were amputated. He recovered nicely and quickly but when gangrene set in his good leg he refused to go back to the hospital in fear they are going to amputate more. I have pleaded with him, but to no advail. His baby toe looks like its about to fall off his entire foot is black. He had lost probably about 80 pounds and looks no more than about 110 pounds. He refuses to eat much but drinks soooo much. Today he looks really bad, really week and says both of his legs are in extreme pain. As he is sleeping I see him twitching and his whole body sometime jolts and shakes. Are his organs shutting down? Is he surming to this infection? I can not see him suffer and I told him so but he refuses to go into a hospital. How long can one survive with untreated gangrene its already been about two weeks that it has turned black.