Just a tip - he will need a PCR for Canada 72hrs before departure. We have randox walk in clinics in NI, but not sure they are anywhere in GB close to you, and all non-US/Canadian arrivals in YYZ/YUL/YVR are supposed to be tested on arrival in Canada and then quarantine until that test result comes back (he could be tested at the airport or more likely have to take a self-test to his final quarantine address and a courrier will pick it up). Result, from what I've heard, can take 2-3 days.

I'm travelling to Canada next week and have set up camp in a hotel in Montreal for 3 nights post-arrival to await result.

However, there are some exceptions for transit through to the US. As your Dad will be entering Canada, I don't know if they will apply - it may be better to fly. I am also not sure if he can put a quarantine address in the ArriveCAN app (mandatory for canadian arrivals - otherwise boarding may be denied) which is in the US. You'll have to check the exact requirements for his quarantine period post-arrival test at travel.gov.ca

Connecting by flight may offer a better opportunity to avoid all of these measures as if you have a connecting flight you are exempt from arrival testing requirements. I emphasise *may* as ultimately it's the canadian border police who make the final decision, but in order to be allowed to board in the first place he will certainly need a PCR test and a Vaccine Certificate recognised by Canada and he'll need to fill in the ArrriveCAN app on his phone (though the last point can be done on arrival in Canada I think).

Just be sure to read the rules on the Canadian government site super carefully to see what applies to his situation when he is driving cross-border, seemingly straight after landing in CA. I'm not sure it's permitted given all the fluff around arrival tests.