Sorry I have not been in the forum for the last couple days or I would have had a few comments during your troubling times. I am glad he is better. Personally, I think the source of the infection was at the 'point of sale'. From your description it sounds like there is some type of large puppy sale once a month. If I am correct and lots of puppies from all over are brought together for a weekend sale once a month....that is the source of the infection.
If I have read this situation correctly I would advise anyone buying a pup to try to avoid these types of sales. Try just going to a private breeder.
Parvo is a nasty virus. it is debateable how long it can survive in the outside environment and I would be surprised if he picked it up from your yard if it is fenced. Puppies are vaccinated frequently during the ages of 6-16 weeks for the disease because you want to stimulate their developing immune system at the precise time (if possible) that the immunity given to them through their mother's milk is waning.
It is thought that Dobermans and Rotweillers are particularly sensitive to the virus and many people will actually give these breeds another vaccine at 6 months of age.
The virus generally causes GI problems but has been know to cause heart damage in puppies as well. This is difficult to detect, and luckily, rare.