By 8 weeks most dogs testicles will have descended, it's fairly common to have one retained testicle which takes a bit longer to descend. What my vets do with retained testicles is wait until neutering age and if they / it hasnt descended they neuter. Neutering a dog with one or both retained testicles is more tricky than a usual castration but they need to be removed as retained testicles often become cancerous (later in life).
Basically whether or not they are going to come down you would not want to operate anyway until he was fully grown.
Don't let your vet convince you to have him castrated young or start giving him injections.