Any dog of any age entered in regular AKC conformation class for championship points MUST have two descended testicles in the scrotum. Puppy classes do not matter, nor does age, the puppy will be disqualified if he goes into the ring with only one testicle.
There is a new 4-6 puppy beginner puppy competition being held at some shows, but there are no points awarded towards Championships, so while they will not be disqualified, they do not get a chance towards points, it is for Certificates of Merit. Again, no points for Championships, so gives puppies ring experience.
Any alteration to improve a dog including surgeries to "fix" something is illegal according to AKC rules.
Bringing a testicle down into the scrotum can be done, but the testicle is dead and won't rejunvinate its sperm. It would be a very prolonged invasive surgery, it would be abdominal surgery to find the testicle, try to stretch the cord or implant a ligament in its place to stretch it into the scrotum. The ring would be already closed, so that would also have to be cut and the testicle pulled through.
It would also be very very unethicalof ANY vet to do this so the dog could show. There are vets that do anything for money, however these vets are not exactly what I call ethical.
There is a shot that young puppies can be given before 12 weeks old, it is very painful and takes three or four shots to try it. I admit I did it one time for a puppy because he was going to be a spectacular show dog, that puppy cried and screamed during the 1cc shot in the muscle. I beat myself up about it, I had no idea it was so painful, just because I wanted him to be a show dog. He never got another shot, the testicle never did come down, he was neutered and became a very special pet.
Enjoy your dog