Would my dog show signs of improvement if he has parvo?

We have a 6 month old basset hound. Two days ago when we woke up we noticed that he didn't want to get up and walk, he was fine the night before, he was vomiting and had diarrhea. The vet told us they think he has parvo and that we should bring him in for an $100 initial examination and then $400 worth of treatments. He had a parvo shot when he was a puppy. Yesterday he was still a little sluggish but was up moving around a little more. Today he has shown a great improvement, still not eager to eat but he has drank water, and walked around quite a bit. So my question is, if he really has parvo like the vet suggested would he be showing signs of improvements with no medications.