Wow... id be whipping my puppy out of there and taking him somewhere else. If a vet does not sit down and explain everything to you in a way you can understand it (not medical talk that makes no sense) then there is something wrong. Either they are making things up to keep him for days cuz they need $$ or whatever other reason, but not bc of the pup itself. If this is the vet that works in conjunction with the pet store, shame on them for trying to blame it on you, but also just typical behavior on their part to cover the store's A$$!!
I think like a hospital for people, a person should be able to walk into a animal hospital where their pet is admitted and sick at any time during the day and be able to sit with their pet for however long they want to! I bet there is nothing more healing to your little guy than to be held by you while hes so sick!
Do yourself a favor and after this fiasco get yourself a new vet that you are comfortable with and that you trust and like a lot. This vet sounds fishy & suspicious!!! I hate that!
I hope your puppy is doing better!!