Why won't my puppy open his mouth?

i have three puppy english mastiffs that i inherited from a lady down the street. i was supposed to get one thursday but i got him last night. she gave one of the 7 puppies away to a new home. three of the puppies died of a three day span. now i have three sick puppies in my basement. we took them to the vet and the puppy that was supposed to be mine, Louie, has parvo. the vet hydrated all of the puppies and gave them diharrea medicine. he is pretty sure all oof the puppies have parvo. he said that louie's mouth specifically was setting and that he was gonna die soon. its been over 12 hours and two of the three are looking better. but louie's mouth is very hard to open. i have to pry it open to give him electrolyte water. is there anything i can do for my puppy other than a vaccine and electrolyte water? does this (his mouth is starting to "set") really mean he's going to die? is there a way to reverse this effect?