Others can comment as well but does not sound a strong case especially given never hospitalised so his condition is managed with medication.. Not saying you could not try with that if you get reported on coming back but does not sound like a strong case but just one of those things everyone faces when leaving their home country with parents left behind..
Also note that PR card validity is not relevant to the residency obligation, you have to show 2 years in the 5 years from initial landing or after the initial 5 years 2 years in the 5 years preceding any new entry.
Just ensure you return before your card expires, although given10/2020 plenty of time, given once your card expires you need a PRTD which for sure having not met the RO could result in PR being revoked