Your son is still at a fragile stage where the prescription is still neutralizing. If he was diagnosed as having -7.00D of myopia, as he gets older, that myopia will decrease because the structures in his eyes which contributes to the prescription. If this prescription stays until he is about 9-10yrs of age, then you might want to get an a-scan to make sure that he does not have pathological myopia
Also, the main reason why he is taking his glasses off to read is because his rx is changing which is helping him see clearly at near, hence the term nearsightedness
, without his glasses. This is very common in myopes.
Hope this helps.
Dr. Jay Patel