If it were me, I'd have him back with his momma. He is way too young and small. A three hour drive would be with it to ensure he is getting what he needs to survive.
If you are not going to do that, then you need to get him into your vets ASAP. He needs to be treated like a puppy that has been orphaned. The vet can give you all the info on what to do.
Grrrrr, greeders like this frustrate me.
I wish you and your little guy the very best of luck.