So sorry to hear that Gunner has parvo.

Your vet is right, he will most likely become dehydrated over the next few days and the one thing that I would suggest is to start SubQ fluids now. Not constant but at least once a day.
I'm assuming that since you were given a bag of fluids already that your familiar with doing SubQ? Not sure how much your pup weighs but 25-50 ml of Normosol R sure would lessen the dehydration. Just my opinion and
NOT something that you should do without first speaking to your vet as well as being versed in or taught how to give SubQ.