Get him out with you, carry him if you have to, lots of car rides, high streets, out at other people's houses with their dogs, socialise socialise socialise is my advice.
Parvo is nasty, but the chances are that the puppy may have picked it up prior to arriving at his new owners, Parvo is still rare, it is scary but we can't wrap them in cotton wall.
Take precautions but make sure he is out with you as much as possible.