Both were present at 8 weeks and at 4 months when at the vet.

...yes, ok...so you might find this well weird, but with Shibas can to be...

and also for other race Puppies testes go up and down for quite a few months, but assuming he drew it back up

(because it's not arbitrary but an action) and it never came down again, who knows.... there are several mechanisms by why a dog ends up with 1 testicle in his scrotum.

Add another to zero testes in the scrotum it is not only genetics!

I'd be inclined to blame that magical 'closing of the inquinal ring' even though in all reality it's not a magic ring, nor does it close like a door..

You can massage his testicles and help them come down, from what I was told from japan Breeders you just gently massage the testicles downwards...

Put him on your lap with his back against your belly-

take your thumb and put it at his groin, above the scrotum- put your other fingers on that hand below his scrotum. Manipulate your thumb towards the scrotum with little dipping movements and try to find the missing one. If you feel it, like a pearl, quit dipping and start trying to maneuver it into the scrotum. Once there, plan to stay there for 10 min or so, holding the testicle where it should be. Repeat daily at least once.

Which side is down?

if it is the right you have a decent shot at massaging the left down- if it is the left, that cord is longer and the saying

(breeders saying)

"if only the left is down, they are right out the door' would apply...the right probably will not descend of new"....

In case if your dog it is a cryptorchid from the beginning the breeder must tell...

Not forgott in any case a having him neutered before 18 months is more dangerous then the Cryptorchidism.

Neutering to young can cause a bunch of things later on in life.