But what would you do if he had remained in Canada and got drafted into their army? A country's own laws apply in their own country. If a man cannot be claimed under another nation's laws, he cannot be made to serve in that nation's army.
Like I said, Canada applies its own laws in its own ways.
Now if Canada attempted to force him into its army, he might be able to claim exemption on the basis that he was also an American citizen. But whether that would be accepted or not depends on Canadian law, not American law.
Then why do male resident aliens have to register for Selective Service at 18 years of age in the US?
WHO MUST REGISTER
Almost all male U.S. citizens, and male immigrants living in the U.S., who are 18 through 25, are required to register with Selective Service. It’s important to know that even though he is registered, a man will not automatically be inducted into the military. In a crisis requiring a draft, men would be called in sequence determined by random lottery number and year of birth. Then, they would be examined for mental, physical and moral fitness by the military before being deferred or exempted from military service or inducted into the Armed Forces.