This is likely due to a weakening of the red and blue guns.

Most sets have controls to set up the picture tube, to adjust for normal
sample-to-sample variation. Assuming your set has these controls, you can do
it yourself. But you have to know how to do it. Once you know what's
involved, it's trivial, but it takes too long to explain it here. Perhaps
someone can point you to a Website with instructions.

PS: Your son should not have a TV in his room. Toss the set.