Your son is having refractive error
in the form of myopic astigmatism
Regarding your first question, the refractive error especially in children is known to undergo frequent changes because of rapid development of the eye. However the chances of it becoming nil are not there.
Regarding your second question he has to take diet rich in vitamin A
and green leafy vegetables better healthy eyes.
Regarding third question, watching TV is not a problem only thing he must practice healthy ways like sitting atleast 6 feet away from TV.
Regarding your fourth question electronic gadgets if used in unscientific manner can cause eye strain
, like using them from too close or overusing them. Hence he has to use electronic gadgets following all the clean healthy habits.