My puppy is one next week, and his testicles appear to be in the scrotum most of the time, but when he lies on his back they are still on occasion visible in the abdomen just above the scrotum and then they disappear again! I really did not want him to be castrated, but I'm sure this is what the vet will recommend...opinions, please!

anyone else experience of this? Thanks!

You do not say what breed but normally if they (the testicles) have made their way as far as the scrotum then they will stay the right side of the pelvis and there will be no problems and no need to even think of castrating. If you stand your dog on his hind legs and get someone to hold his front legs so he is stood upright the missing jewels should be in their correct place, if they are all is well.
When the testis come down from the abdomen they pass through a narrow gap and then make their way down till they end up in the scrotum, once they are through the narrow bit there is not usually a problem with retained testicles.