Hello, I am Dr. Peter a licensed Veterinarian. I would like to help. In response to your question, if you do not find the testicles in the scrotum then it means he is either neutered or is cryptorchid (testis never descended into the scrotum).
Try to look for a scar in the bottom side right in front of the scrotum (pre scrotal area); you may find the scar from being neutered. If you can not see a scar, than he will need to see your family veterinarian to check hormone levels and make certain he is neutered.
Hope the above helps, if you have additional questions I will be glad to answer them.
The scrotum atrophies if the dog was neutered as a puppy.
The scar incision should be in the middle right in front of where the scrotum would be. Put the dog on his back, open his legs and right on the shaft of the penis in front of the scrotum is where the scar incision should be.