For your peace of mind why don't you submit an ID which is listed as an example on the website? It can be very painful waiting for AORs for months and asking yourself if your application will be accepted or returned because of providing a PR card as an ID for your son. When I applied as a family last year the website specifically said a PR card cannot be used as a piece of ID for minor. Looks like the website was updated and they don't mention that information anymore. Don't you have any others IDs for your minor?
Application has already been submitted with PR card as a piece of ID. Since the instruction guide does not explicitly prohibit that and a PR card falls perfectly under this requirement
“Photocopy of two (2) pieces of personal identification.
Both pieces of identification should show the minor’s name and date of birth, one of which must have a photo on it.”
I really don’t see an issue with it. I guess time will tell, or somebody that has first hand experience with this after apparently, the rules were changed.
P.S if indeed it is not an acceptable form of ID it is extremely irresponsible of IRCC to not
explicitly mention this in the instruction which is literally the only official guide to anyone going through this already stressful and complicated process