Welcome to HCM.
Children are prescribed glasses to correct refractive error
. Your Son's power suggests that he has Short sight/Myopia
,where in,his eyes are slightly larger compared to a normal eye, making it difficult to focus things clearly. So glasses are given , through which he can enjoy his full vision without straining his eyes.
Since most of the children do not inform the parents about any change in their power, it is common that they are advised to have a checkup done once every 6mths.
And as the child grows his power varies according to the growth of his eyes. Increase or decrease in power varies between individuals. Don't worry,he is enjoying his vision like a normal person through his glasses.
As long as he is comfortable wearing glasses, encourage him ,so he maintains his full correction. Once he is 13-15 yr old he can try using contact lens
and later above 18 yrs/ once his power stabilizes he can go for permanent cure through LASER
Hope this helps.