Does "non essential" apply to air travel ? From what I have seen it's only for Land travelers. i.e. a Canadian or a PR can fly back to Canada from US (Fact) but they cannot drive to Canada from US because they would not be classed as essential.I am not sure you will be allowed to complete your landing. They will see you as non-essential travel.
I think same applies in this case the only thing I am not sure about is the self isolation part but I guess if you are not leaving the airport and are flying back the same day then maybe ok ?
He might however be subject to quarantine once he lands back in the US as he's coming back to stay.