
Your breeder has probably told you the 100% truth, but if you bought a puppy that had 2 testicles on purchase, then one retracted, how many more could of done that? and how many people stay in touch with the breeders of their puppies? They in all reality do not know how many monorchids they have bred, likewise for most other breeders who do not keep in touch with EVERY male puppy they sell, its not realistic.
I think you are right to want to show him, but you will get knocked, monorchidism is a BIG no no in dog showing, whilst castration is not favoured, you stand a far better chance with a "normal" looking dog, than one that is obviously not. Strictly speaking, who is to know your dog only had one testicle before?
Testicular tumours are a very real possibility in retained testicles, they are at a much higher temperature inside the body than they should be outside of it, ultimately castrating him is the only thing that will prevent this.
If you continue to show him, I wish you the very best of luck, and you are right, we always bring the best dog home!
Dawn.