He may not have needed them before, and it came on slowly. Their eyes are pretty adaptable when younger. Kids don't always know how to articulate the change. Our schools screen vision each year, but you should still take them in for a thorough exam every two years. Even going in yearly or more, I've wondered how we've missed things -- like, how long did my DS need a bifocal addition before we caught it?
From what I understand, each year as they grow, more and more students are identified as near-sighted. Where you'll see few kids with glasses in K, more will wear them each year. Some studies suggest that increased indoor time is a risk factor (the more the schooling, the more likely to be nearsighted), and spending more time outdoors (whether it's sunlight or looking further away, I'm not sure) can reduce the development or progression of myopia.
