Does mega cistern magna and arachnoid cysts in left cerebellar region indicate a serious condition in a 7yr old boy who has no symptoms at all ( he has complained of being dizzy 2-3 times in his whole life time.. but nothing other than that)?

Based upon the provided information, there appears to be no rationale for current concern, as you describe congenital anatomical variants. Over time, if neurological symptoms emerge and progress, then actions should be taken. But at this time, am dismayed that this type of imaging was obtained based on infrequent complaints of dizziness in a normal child!