That is something you need to talk with insurance providers because with a pre-existing condition he will not be covered for many treatments, if required.
Best to assume it won't be possible to cover any treatments related to the cancer or any other medical expenses related to this. If he will require any treatments while in Canada, assume you will need to pay for this out of pocket and look into the costs in advance, as well as a clinic that will accept him as a patient.
Good chance he will not pass the medical so that will also be an issue. Provinces are required to prioritize care of their own patients and there is not extra capacity in healthcare these days. Some hospitals don’t accept foreign patients for non-emergency treatment.