I have been following this thread closely Cyn. I am so sorry to hear this terrible news.
I don't know if you know who James Herriot is, but he was a country veterinarian in the thirties. I have read all of his books, and one of the things that he did a few times was "put to sleep" an animal, but not in the way you are thinking. If he didn't think that an animal would make it as it was, but that if given a chance to heal it could live, he would give it enough sedative to keep it asleep for a day or two, and keep giving it sedative every time it began to awaken for several days. Both of the times he documented this, the animal recovered. Is there any way you could get your hand on some sedative? Something that you can inject and knock him out? Talk about grasping at straws, but maybe worth a shot?

