Sorry to hear about little Frankie

Very dismayed to hear he was sold at just 6 weeks of age, it's far too young to be sent out into the world especially as he was so small to begin with.
I hope he makes good progress and I trust your vets are being sensible and will not even consider vaccinating him until he is much older and well over his current problems as this could push him well over the edge. Get him better and get him home, build him up and take him back again when he's fully fit before you have him innoculated, there'll be plenty of time to get him out on lead walks once he's well enough. I would also check with the breeder to find out if any of the other pups have had problems, it could be that the bitch was carrying a bug while she was pregnant and might have passed something on to the pups, campylobacter is often the cause but your vets will be able to tell you once the results are back.
Get well soon little Frankie
Sandy