I've never fed my parvo pups until the first 48-72 hours is past and they show interest in eating on their own. If they're that down and out (and they can be nearly comatose and still pull through), I just keep in mind that they can go weeks without food, but need electrolytes and hydration. I give them sub-q fluids several times per day, hourly enemas with a special tea I made, and plenty of electrolytes via Pedialyte, but their guts are going through an awful lot and I don't think digesting food helps. The body needs to fast to heal. Their intestines are shedding...food isn't what they need. They need rest and fluids and meds to prevent secondary infection.
I use Parvaid and make tea with it also, and between the enemas and sub-q fluids, I haven't lost one yet (knock on wood). I hope I never have to do it again, but I pulled 3 through with no crazy vet bills.
When they're better, I start with little bits of cooked liver and white rice, canned chicken and white rice, progressing to ground beef and rice and a probiotics until they're back on track.
Best of luck to your little pup. Visit him often! Depression and confusion at being in the hospital will not help, so the more time you can spend with him, the better. I cuddled mine a lot. It seemed to help. My vet hates to hospitalize parvo pups; he firmly believes if the owner is at all medically-savvy, the pups are better off at home under their owner's care and following his instructions than in an unfamiliar place.