You already addressed that he takes care of his musculoskeletal health by sitting the right way and that he has a 6 pm curfew for screen time, so no sleep problems either. Vision problems are likely, but personally I get headaches more often. I mean, my visions been bad for years and it takes a while for screen-time to have a huge effect on that.
1. The retina contains three kinds of colour receptors
Explanation:
Young-Helmholtz theory color vision says human can perceive colour due to the presence of colour receptors in the retina that receives colour in various wavelengths