Everyone over 12 is charged the adult fare.
As to the drinking issue... wherever you travel, it's always a matter of "When in Rome." Since Disney's primary market is the U.S., they're not going to buck U.S. practices. Your son isn't going to be treated as a full Canadian adult anywhere south of the border. It's not something peculiar to Disney. Even on Carnival Cruise Line, with its reputation for partying, the drinking age is 21.
When I was 18, the most common drinking age in the U.S. was 18. When the voting age was dropped from 21 to 18 by constitutional amendment (back in the early 70s), a lot of states followed suit with drinking age (some had been at 18, some 19, some 21). It was the Vietnam era, and "Old enough to fight, old enough to vote" (and drink) was a popular sentiment. In 1984 (oh, the irony) the U.S. passed the National Minimum Drinking Age Act, which withheld certain federal funds from states that didn't raise their drinking age to 21. Such is life in Rome.
Yes, in the vast majority of nations around the world, 18 is the legal drinking age, and in most, drinking below that age is allowed with some form of parental supervision - even that isn't allowed in 15 U.S. states.
Co-Author, PassPorter's Walt Disney World, PassPorter's Disney Cruise Line, and PassPorter's Disneyland and Southern California Attractions