I was thinking the same thing as Dottie--it sounds like he doesn't want to be a big boy. The fact that he was reading silently when you weren't looking and he stopped when he knew *you knew* he was reading makes it seem like this is more about your relationship with him than any actual problem with reading itself.
Has something changed in the home? New baby on the way? A friend with a new baby? Something diverting attention from your son?
I could be way off the mark, but that's where I'd start looking if I were you.
