My corgi Brian had one undescended testicle which had still failed to drop by the time we took him in to be neutered (he was 6 months). They removed the descended one normally, but for the undescended one, they made another incision and had to "search" a bit for it. It took a little longer for him to heal at that site, but he was fine after about a week. They did the procedure with lasers, but I'm not sure if that had any affect on it. I would just say find a vet that has experience with neutering on dogs with undescended testicles.
Hi Ducky, my Charlie also had undecended testicles. The neuter is more involved than a typical neuter because they have to open them up to find the testicles. He had quite a bit of bruising and because the incision was right next to his penis, even the skin there was bruised. He was on limited exercise for about a week and did not feel like playing, so that was ok with him. I did wait until he was around 2 to have it done so he was over the hyper puppy stage. I was worried also about the surgery but it would be best to have it done because the chance of testicular cancer is greater if their testicles never decend.
Dont worry so much - if your vet is experienced he probably has done this before. It is more common than you would think. Let us know how he does. I bet he will be just fine. (by the way, my nickname is "Duckie")