Parvo is more than being dehydrated, it's an intestinal disease which doesn't allow nutrients to be absorbed. Giving your pup water is good but he probably won't get re-hydrated because the parvo prevents that from happening. The only thing you can do is get him to a vet for treatment. Even then the survival rate isn't great.
Here's a link about parvo;
Parvo | Dogs - Health, Training, Breed Info and More
This really isn't something you can treat at home. I wish there were other options but there really aren't. The vet is the only option, so please, find a way.