I would hold off with the castration, it still may come down.
A friends cocker didn't have his 2nd testicle descend until 12 months, so don't give up hope.
Personally, i wouldn't let the vet persuade me to castrate him.
Usually the breeder do get the puppies vet checked just before they leave for their new homes, and this would have been noticed at his check. Do you know if this was done? The vets do keep records of the puppy checks, so if this was the case then you would have cause to speak to the breeder.
Another point to make though, is at 8 weeks there are no guarentees that a puppy would mature to be good enough to show, and it really is a sort of gamble really.
There are still fun shows, agility, training, and you could still take him to ringcraft even if you don't show him. I hope you love him for what he is though, and if your plans to show him don't succeed, perhaps try another activity. Personally, i wouldn't think its that important, but i can understand your dissapointment if you've had your heart set on showing.