The reason this was so hard to figure out is because his right eye is 20/20 while his left eye is farsighted at about 20/60. He was using both eyes together to focus, and thus had been doing just fine seeing things. But his eye and brain sometimes had to work really hard to focus, especially when focusing on the same thing for a while, like a movie, hence the headaches.
So now he is wearing these cute glasses. He is doing really well with them. They are mostly needed for home and school, so he doesn't need to wear them when he's on the playground or other situations where they could get bent or broken (whew!)
I think he's quite handsome in his glasses!