There is no absolute time that a child should be retractable. I have seen information that runs the full gammut from 3 years old to late teens. The best information I have seen is that 2/3 are retractable by 6 years old and 90% by 10 years old and some are not until mid-late teens. I can't remember where that was but it can probably be found in the Circumcision Information Resource Pages (CIRP). The only value to that document is that there isn't a normal time. It is not essential that the foreskin be retractable by a certain age. There was a wife/mother here about a year ago whose 32 year old husband had NEVER retracted and wasn't concerned about it and had little interest in retracting. Obviously, reproduction wasn't a problem since he is a father and he was satisfied with his sex life and there wasn't any problem with hygiene since his wife had no concerns.
If the child is not retractable by mid to late teens, the foreskin opening can be gently stretched using betamethesone cream .05%, a steroid ointment. The stretching process takes a couple of weeks to a couple of months and is 99% effective.
You don't have to prove your point to the doctor. You just say "no!" and that's that. They can not circumcise your son without your approval. If they start giving you too much grief about it, just go to a doctor who is foreskin savvy. That may not be a bad idea anyway.