Hello, this is Mrs. Idaho-ar15.
The main symptoms of Parvovirus infection are vomiting, a REALLY nasty bloody diarrhea, extreme anorexia (from nausea), and severe depression.
If the puppy is still alive it's very safe to assume he does not have parvo. If he's eating at all, he probably doesn't have parvo. If the diarrhea is not squirting out of him and bloody, he probably does not have parvo. And if he's not laying there like he wishes he would die, he probably doesn't have parvo. (emphasis on the probably's - sometimes they surprise you!)
My guess would be that the pup ate something while he was homeless that really didn't agree with him.
Most dogs that get it and are not treated die from dehydration within 3-4 days. Most dogs that get it and ARE treated survive. We save about 75% of the pups that come in with it.