My DS also had the same problem on long car journeys, so we were always armed with bucket, cloths, wipes and a change of clothes etc....
until we discovered these:
Traveleeze Soft & Chewy Pastilles - 10 Pastilles
http://www.boots.com/en/Traveleeze-Soft ... les_17939/
They are suitable from the age of 2!
Worked a treat with DS.
On one occasion we did forget to give him it and he did throw up.....so they were clearly working otherwise!
They are the chewy type so he just thought it was a sweet any way so no probs taking it