Is your son’s glasses RX - (MINUS) 3 in each eye or + (PLUS) 3 in each eye as you wrote it both ways. In any case there is no way to know what his glasses RX will be when he is an adult anymore than you can predict his height or weight. Stick close to a pediatric ophthalmologist Eye MD. Also be sure all other children are checked as this runs in families and all children at risk.