The short answer is he would either need to get a travel document at the Canadian Embassy in Mexico before traveling back after landing or travel to a US border town (with a US visa) and drive into Canada.
The rationale is a bit convoluted, so bear with me:
The stamped CoPR is proof of his status so if he could make it to the Canadian border he would be allowed in based on the CoPR. HOWEVER, the CoPR is not a form of travel that the Canadian government authorizes and trains commercial carriers to use/verify to confirm an individuals's admissibility. And, commercial carriers take the responsibility to ensure you have a valid travel document (e.g. visa-exempt passport, visa foil in your passport, PR Card, Canadian Passport) very seriously because if they allow someone who doesn't have a valid document to arrive on Canadian soil and the person isn't allowed in the airline (or carrier) would face a very stiff penalty.
Therefore, the Canadian government allows you to get a travel document to prove your eligibility to travel to the carrier to be able to travel back to Canada.
And, this is also why folks who are visa exempt can travel without a travel document (the airline is covered by having them be eligible to enter as tourists) and why folks in the US who can get to a border crossing themselves (walking, private car, rental car) can enter with their CoPR.
BTW, on mailing PR cards, I know people do it but, FWIW, you aren't supposed to. If Canada Post or the courier find one being mailed abroad, they have instructions to seize it and send it back to CIC. Not a headache I would want to sort out.
