A layover does not need to be listed in the foreign travel section if your son did not leave the airport. If your son had left the airport for any reason to travel within the country, then yes, it should absolutely be reported even if he only left for an hour or so. If your son wants to fully cover his bases, then i would have him list in the comments section under his UK travel that he had a one hour layover in Canada where he did not leave the airport or travel at all within Canada. That would cover any honesty issue should a reviewer or investigator take issue with his reporting.
I experience instances all the time where during a review of an individual’s passport, stamps from several countries appear in the passport that weren’t listed in the comments. In this case, all that is really required os an explanation that the passport stamp was in relation to catching a connecting flight and no actual travel occurred within the country. I have never had any of the Federal agencies take exception to this reporting style.
As a heads up, you may get different answers on this as it isn’t specifically noted in the instructions or guidance. My knowledge of the topic comes from a half decade of experience and clarification from the government agencies.