Question is that the vet recommended from where you got the puppy? If sometimes they might not be completely straight with you because they don't want you to think you got a bad puppy. Take the puppy to another vet and if you could go to an animal hospital.
Since the vet you took him to didn't seem to think anything was really wrong, and since there obviously IS something quite wrong, if it were me, unless this is a vet that you have used for many years and trust implicitly, I would take him to a different vet. Whatever is happening either WASN'T happening when your vet examined him or WAS happening and your vet completely missed it, so you need to be honest with yourself and decide which one you think it was and act accordingly. If you think the vet made a good call based on how the puppy was acting during the exam then take him back to that vet and let him re-examine him now that he is truly acting sick. If you left the vet unsatisfied with the diagnosis, knowing deep down that something was wrong with your puppy and the vet just didn't pick up on it, then go to a different vet.
With regard to it being pancreatitis, it's very unlikely that your puppy would develop pancreatitis from ONE piece of hotdog. Pancreatitis is caused by the dog eating something very high in fat, and a bite of hotdog, while it CAN cause stomach upset, particularly in such a young puppy, shouldn't be enough to do anything at all to the pancreas unless the organ is badly damaged in the first place. Something else to remember about pancreatitis is that in a healthy pancreas, even a meal that is high in fat is not likely to cause pancreatitis, but many dogs have unknown pancreatic issues and that's why they develop pancreatitis. In other words, the fat doesn't cause the pancreas to go bad, but it will flare up an already unhealthy organ.
There is something much more going on here if your puppy will not walk. Please let us know what the vet says when you take him back, or what a different vet says if that is the route you choose to take.
Ghilly
He needs to be seen by a Vet ASAP, either the one you went toor see a second one.