Has your dog been examined by a Vet? Was it determined he only has one testicle, which is not common. If the other testicle did not descend into the scrotum, it could be the other testicle. A retained testicle should always be removed! It can cause problems in the future and possibly become malignant.
This could be a hernia of some sort.
You should really have him examined by a Vet and get him neutered.
Good luck and please update
I would get him checked out. It could be something like a hernia, or less likely because of his age, a mass among others.