No experience of parvo either but when Coco was a tiny pup she often had runny poo
Have you changed his food lately? Or has he eaten anything he shouldn't have out in the garden? It might just be a tummy bug but I would def speak to the vet if it lasts more than 24 hours as he is so young and they can dehydrate very quickly
Starving him for 24 hours and then a few days of chicken and rice with some live yogurt then gradually changing back to his normal food might do the trick but I would talk to the vet anyway
There are loads of things other than parvo it could be though
Hope he is better soon