Sooo sorry to hear about your baby's illness - I hope he pulls through and this all becomes a memory very quickly.
Something you wrote did ring a bell however, and that seemed to suggest that Toby was unvaccinated.
Puppies can be vaccinated at 6 weeks using Nobivac, and once again at 12 weeks. This helps protect pups re-homed at (their optimum age) 8 weeks.
What ails him may be absolutely unconnected with the usual parvo/distemper etc. but all puppies should have had at least their first vaccination and a health check by a Vet. before they are re-homed. Most breeders (and us at the Blue Cross) give 6 weeks free PetPlan insurance as long as the animal is 8 weeks old and has no pre-existing condition which could be excluded from cover at PetPlan's discretion.
This is all absolutely no consolation or help to you at the moment, but you may have to question Toby's health when you first picked him up, and if there is anything the breeder can tell you which may narrow the search for a cause, then you need to keep in touch with them, and also the other new owners of the rest of the litter (preferably with them and not with the breeder as an intermediary).
Here's wishing the little mite (and you) all the best - keep us posted ...
