Are you prepared to risk having to give him back?
Was he sold as a stud dog?
Or just a family pet?
You could try to argue that this was a misrepresentation.
A misrepresentation can be innocent but it isn't usually absolute. The reality is that at the time of sale it is not likely that this fault would have developed which does not really get over the hurdle of misrepresentation.
Failing that you could drop down on the law of contract and demand that this is not a dog 'fit for purpose'.
The consumer rights act wasn't really written for livestock.
However, livestock is property and subject to it.
It is obviously harder to say what is a fault though as no pedigree dog is perfect.
The usual argument from breeders is that they weren't expecting people to want to breed or show but were selling a family pet.
if they were told that your purpose was to show him then that would be removed from them.
Then it would depend on whether the fault had developed at the time of sale or that there was a propensity.
I think, overall, a very substantial vet issue in a very young dog would probably get over the hurdle.
Generally speaking though courts are more sympathetic to people who have bought family pets than others. That said, they don't much like breeders either given the overpopulation in the UK of companion animals and the misery to dogs it causes so I suspect the two cancel each other out.
Can I clarify anything for you?
Do you have their address? Don't give it to me but do you have it?
You should really start with a letter before action warning them that you will issue.
The problem with this may be the future of the dog. They are unlikely to want him back.
All the best.