William had exactly the same problem, an undescended testicle, place of hiding unknown until they operated to remove it. It was near one of his kidneys and apparently it took the vet a while to find it and remove it, which was done when he was 15 months old. I waited until then in the hope that it might appear. He had 2 lots of stitches from the removal of the undescended testicle. One lot was internal, the other external. I can still faintly see the scar from the external stitches.
He had the other testicle removed at the same time, so is fully castrated. I sometimes regret that, as his coat is nowhere near as nice as it was. He's a show type and, as I used to show a bit, I like to keep him in something resembling show trim. But that's just a bit of an annoyance to me, not a problem to William.
After the op. he felt very sorry for himself for about 24 hours. I think it was partly stress from having to spend a whole day at the surgery, which was a new experience for him. It was a new experience for me too, seeing my happy, confident boy in such a frightened state. It was also certainly partly due to pain. I took him back to the vet the next day and got some additional pain relief for him. He was a lot happier after that, but I still ended going back a few more times as he had some swelling and fluid behind the external stitches. I was worried about that, but it didn't bother William at all and it didn't require any additional treatment.
He had to wear a lampshade at first and really hated it, so I decided to just put it on him at night or if I went out and to watch him like a hawk the rest of the time. That worked well and after a few days I think he'd forgotten about the stitches and they clearly weren't bothering him by then.
My insurers are NFU and they flatly refused to pay up because it was elective surgery. I wrote to them and pointed out that it was done on the recommendation of my vet, but they wouldn't pay up.
