Thank you for your question.
I see you say the dog was tested for parvo - was that before this all started to happen, when you got the pup? or was it after?
Given the dog's age I need to know more information to help
Mostly - is the dog drinking and eating? if so what is are you feeding him? Where do you live?
It sounds like it could be a few things- let me know and I will help you get to the bottom of it.