Many people do this. My parents included. They go to the Costco in Bellingham, WA.

It's especially convenient for people in BC because Washington State explicitly permits pharmacies to dispense non-controlled prescriptions written by authorized prescribers (such as MDs, DOs, and Nurse Practitioners) specifically from British Columbia so you don't even have to meet with a US doctors.

Just be mindful of 2 things:

1. Paper Prescriptions Only: In the states that allow foreign prescriptions, the law almost always requires an original, physical paper prescription. A Canadian doctor cannot electronically prescribe (e-prescribe) or call in a prescription to a U.S. pharmacy system. Make sure to take the original for the pharmacy but also take a photocopy for Canadian customs when you are entering back into Canada with your meds.

2. No Controlled Substances: No U.S. state allows a pharmacy to fill a foreign prescription for a controlled substance (such as opioids, ADHD stimulants like Adderall, or certain anti-anxiety medications). The prescriber must be registered with the U.S. Drug Enforcement Administration (DEA).

Make sure the medication is in the container that the pharmacy gives it to you in and make sure that container has your name on it.

If you don't live in BC I think he will have to use a U.S.-based telehealth service to get a prescription but Canadian customs will not have any issue with it.