the rule is you have 3 month period before you have access to the medical card advantages and his status wont change now that he was given the PR ,and this is not a security issue nor a change in Martial status nor a contagious case ,so no need to inform the IRCC
The rule is that you are required to inform IRCC of any changes to your application before landing whether it is family composition, health, etc. you aren’t a PR until you have landed in Canada. It will be pretty obvious of you come to Canada and seek cancer treatment that you already knew you had cancer before landing. Would be stupid to delay treatments and start with complaining to your GP about pains/GI issues and then go through the whole diagnosis process again. A GP will start with 1st line treatments first, then refer to a GI who may want to do a scope, etc. That could mean 6+ months delay to treatment. Given Covid it may be a much longer delay.
Wait period to access healthcare and residency requirements to receive healthcare are primarily to stop people from only returning to Canada to access healthcare and then leaving again.
