I had to do some checking around. A relatively large number of Americans joined the British Commonwealth (especially Canadian) forces prior to Pearl Harbor. Most of these transitioned to the U. S. Armed Forces when the U. S. entered the war.
The Canadian Government, unfortunately,makes it incredibly difficult-to-almost impossible to access records (this policy, of course, like our own proliferation of so-called "privacy laws" in recent years, was created by lawyers to generate business for lawyers).
Among other requirements, your subject must be deceased at least 20 years. You must be a close relative and a Canadian citizen or resident. You must be able to document all of the above and they're real nit-pickers.
If you somehow manage to make it through the maze, they very often require that the info be released to you through an agent (read: lawyer).