I agree with the above suggestions. You might want to start giving him Probiotics on an empty stomach, intermittantly with garlic on a full stomach. Cherries, grapes, search the web for healing foods for gangrene. He'll be in our prayers.
Hyperbaric oxygen treatment is about the only treatment that is readily available that may (and that is problematical) save his extremities. Your physician should have no objection to this protocol.
He will be in my prayers.
So long as he is under a doctor's care, there is little you can do except support him in every way. His doctor will know what is best for him. He's been thru a lot and facing even more, I'm sure he is depressed about all of this. Talk to him about things he is interested in, stay upbeat for him. You're in a difficult and frustrating position in not being able to do more for him. But your presence and conversation is good for him. Let him know how much you and everyone else needs him, knowing he is still needed will help. Right now he probably feels he is of no use to anyone and you need to try and nip this in the bud. This is how I would feel in his situation, but he needs to fight, so give him a reason to fight. Ask for his help with decisions in your life, this will show that you need him. I wish you and your dad all the best, and will keep you both in my prayers. Take care.