Probably having the screen too close to his eyes. I know one problem for me is that my screens are just past the point where I can see without glasses, but it's probably not a good idea to use glasses when the screen is only a foot and a half away so I can relate to the parent. My room/desk setup also doesn't allow for me to rearrange this to make my screens closer to not need glasses. Guess I'll see how much this affects me in a few years lol
I've been on computers for lots of time since I was 12 years old. My rule of thumb has always been, keep the screen at least an arm length away from my face. I have 20/20 vision. (Knock on wood... I've definitely increased screen time the past year.)