
A lot depends on how long ago you bought the pup. (In other words would the breeder have known the pup was sick, and was it a problem they could have forseen, or tested the parents to minimise the chances of it occuring in the puppies. In this I mean were the parents tested, hip scored, eye tested etc. Even with the best of care and preventative measures it is impossible to guarantee that a puppy will remain healthy.
If the puppy was misrepresented, and was sick at time of purchase, and this was not pointed out to you, then you may have a case, contacting trading standards may be worth while, especially if the pup was commercially bred.
Have you contacted the breeder with a vets report on the pup, as if the breeder is a reputable one they will wish to guard their reputation.