The Fexofenadine I've just bought for ds was 12+ (otc).
Keep pollen out of the bedroom (curtains shut if windows open)
Cover his bed with a clean sheet to keep pollen from settling on his pillow
Change into pollen free clothes as soon as you're inside and wash straight away.
Shower and wash hair before bed. Everyday.
DRY EVERYTHING INDOORS - if you dry bedding and clothes outdoors, pollen will just land in it.
You need to keep the house as allergen free as possible.