
The Kennel Clubs rules for showing a dog are the same be it Companion, Limit, Open or Championship. There are only two reasons that mean a dog can't be shown and they are Aggression and Infection. Apart from that providing you have not had the natural conformation or coat changed by artificial means you can show your dog. Even if you have had the conformation changed providing you have been given permission to show then you can do so.
This means that you can show a dog with no testicles, non standard colour, wrong type of coat, wrong type of tail or even three legs, mind you if you have your 3 legged dog fitted with a false leg that would need permission.
Mind you whether you would wish to show a dog with a fault is up to you but there is no rule to stop you. In the case of late descending testicles I think you should get your dog into Open shows where the judge will make allowance and it will get the dog used to being shown, nothing worse than trying to show a teenage dog for the first time.
Agree once on show tack he will probably be fine, lots of ringcraft and get him to some show quick.