Sorry for your loss.
For future reference, an animal bite is one of the things that warrant an antibiotic.
Green poo is bile, and an indication that he is not eating, as you saw.
You did the best you could and that is that. Chances are after the shock wore off, he had internal injuries that nobody could do anything about, no matter how much time or money was spent. There is no fixing an internal injury you can't see or even know where to look.  You know he died in loving hands and that is the important part.