I'm very sorry that you're writing about parvo today. It is not a pleasant topic.
I would be thinking positive at this point, if I were you, if he has held on for 6 days so far. How is he doing at the clinic, will they tell you? I would ask about his eating, vomiting, energy level and stools.
Good luck and good vibes for this guy. I've worked in kennels for years and I've seen parvo in probably hundreds of puppies. IME, there were very few casualties, but that will depend on their immune system, how early it's caught, the strain, your area, etc.