Medicine often helps, but i would like more of his past medical history before suggesting something.
To make sure whether child is intolerant to any of the drug or if there is a chance for allergic manifestation.
Next Steps
instead here are some methods to manage it without medicines:
Before Travel:
Light meal only (biscuits, toast, banana)
Avoid oily, spicy, heavy food
Don’t travel on a completely empty stomach either
Natural Helpers:
Ginger candy / ginger biscuits
Peppermint candy
Cold water sips
During Travel:
Seat him in the front seat (if possible) or middle seat
Ask him to look straight ahead, not sideways
Avoid reading books or phone use
Keep windows slightly open for fresh air