It's common immune response at this age, as it's happening recurrently needed Holistic healing approach.
Good that u had ENT opinion
And not serious structural issues.
Ayurveda offers unique approach for such concerns.
Pollution might be one of concern if so maintain Air purifiers on or run only fan of AC may act as air purifier.
Next Steps
Saline water gargling/coconut oil pulling.
avoid Milk cheese paneer n curd or choose Goat milk
Health Tips
as recurrence illnesses hamper physical n mental development so Suvarna Prashan as Rasayan can resolve but consult Ayurveda Dr in ur city