I am so sorry you and your pup are going through this. You have done everything right yet your still in this horrible situation.
I don't know alot about this virus but I attempted to find some information that might help you. It sounds as though some dogs do survive this but not without some lasting effects. Not alot of treatment options but supporting the immune system ranks high for aiding the body to fight this. I would suggest the best possible diet for starters..if he is eating. Pup may need IV fluids to stay hydrated. I am hoping there the members with much more knowledge will jump in on this thread.
I will pray your puppy is a survivor and can beat this. Please aleays keep what is best for him in mind as you try to work through this.