It is very possible for older puppies, even adult dogs to come down with parvo. We recently had a 1 year old husky mix come down with parvo, thankfully, we caught it in time, and she made it. For anysort of strange behavior - we always test for parvo, because there IS parvo on our property - it's a shelter, it will happed. Bandit was vaccinated, but she still caught it.
Parvo lives in the soil, as well as in other animals ***. It's possible that the parvo has been there since you moved in. A pup can catch parvo anywhere, from walking down the sidewalk, to stepping paw in his backyard. Bleach everything. The grass, pastures, everything. Bleach is the ONLY thing that will kill it - not straight bleach though, has to be mixed, I don't know the exact measurments, someone else should.
Good luck to Cheif! As long as they keep getting fluids into him, he'll pull through - pups are lost to parvo when they have no water left in them.