William had only one testicle descended. We waited until he was 15 months old and then had him castrated, although I sometimes wish he'd just had the undescended one removed and the normal one left, as the castration spoiled his coat. His missing one was right up in the abdomen, so I'm sure that having it removed was the best option to ensure it couldn't become a problem later on.
As bluegirl says, it can be hereditary, so your boy shouldn't be used for breeding. I now know that it's a fault that is in William's fathers bloodlines.