Vet yes. Most breeders around here are giving me a cold shoulder because i didn't get him from them. I didn't buy him either though. But people will be as people are.
Those breeders are jerks.
There is a time and a place to feel miffed about how someone obtained a dog, and this isn't one of them. A decent breeder would not give you the cold shoulder for trying to rescue a pup this young.
I am guessing the rest of the litter did not make it? Is the dam available, or did she die? This pup would be much better off even if there was just one other litter mate.
I am totally unavailable tomorrow. You need a nourishing formula, probably goats milk, someone here probably has a good formula. And you need to warm it and feed this pup six or eight times a day. Let it sleep all it wants, stay close and give it a teddy bear to snuggle up against. Keep it warm. Keep him clean, but do not be too crazy about bathing him. Instead take a moist warm cloth and clean him up wherever he has gotten into anything nasty -- think momma's tongue.
His eyes are probably open, and he should be trying to walk now, if not walking. But I wouldn't take him anywhere save maybe the vet, and then I would hold him and not put him down anywhere -- weigh him in the carrier.
As for weighing him, do that every day, and make sure he is gaining weight. If not, feed him more. If he will not take more, you may need to increase the calorie content of what you are trying to feed.
For this week, don't worry about socializing him. Don't let any other dogs around him. Touch him all over, every day, and as much as possible, let him feel your warmth and presence.
Have your vet give him a dose of wormer. This should be repeated every two weeks. And at six weeks, he should get his first set of vaccinations.
Good luck with the puppy.
I will try to check in late tomorrow night. Others here can give you better help. I have kept a little one alive but her mother was available. I haven't had to manage a total orphan.