So sorry to hear of the troubles! Hope the little guy makes it!
Several years ago, I had two puppies that had gotten their shots about a week prior to breaking with Parvo. The vet said the shots may have been bad and actually gave the pups Parvo. Here, they send the puppies home with you, since we don't have 24-hour animal hospitals. Around-the-clock, I had to give them injections several times a day, and since they weren't able to hold down fluids, had to actually inject large volumes of fluids just under the skin of the neck, etc. It was touch-and-go, but they did make it through. Keeping fluids to them is absolutely vital.
The Parvo virus is pretty well everywhere. As noted above, it can be tracked in on shoes, etc. Just a trip to/from the vet can bring it into your home (or pretty well anywhere else, for that matter). A similar, but not usually having quite as high a mortality rate, is the Corona virus. Vaccines are also available for that.
The reason for giving a series of shots is you are trying to cover when the maternal antibodies no longer protect the puppy. Sometimes, they drop off at 6 weeks. Maybe 8 weeks. No way to tell, so you give the shots, hoping to 'overlap' the period of while the maternal (Mom's) antibodies are still protecting it and start to drop off, when the shot will take over and protect from then on, not leaving an 'unprotected' time span between the mom's antibodies and when the shot protection kicks in. (I hope that makes sense...)
Good luck and keep us posted!