A number of insurance companies now only issue such communications online via a portal, it's all set out in their terms & conditions. Is there a chance he had a typo when entering his email address when organising the insurance? If he received no emails at all, you can be pretty sure they didn't have the correct email address on file.
They are within their rights to cancel the policy if they didn't receive the NCB in time.
Never, ever send the original NCB, always a copy. Some insurers only provide evidence up to 6 years, others look for more for full discounts so it's good to be able to put your hands on previous ones to easily prove a long claim free history.
Your best approach here is to ensure they have a valid NCB and try convince them to reinstate the policy, removing the cancellation from record. Otherwise your son will have to disclose that cancellation to future insurers and some will refuse to quote as a result.