Well it wouldn't stop me from breeding - if he did get one with an undescended testicle it's easy to sort out, it's different situation to not breeding from a mentally ill dog, or any condition that cannot be sorted out.
Anyway, if what u say about not breeding from him is true, then that wud mean his father should be stopped from covering - if it is a genetic thing then my dog got it from his father, who could get more undescending-testicle dogs, couldn't he?
Sam