Hello, Medications are most effective when taken prophylactically before travelling, or as soon as possible after the onset of symptoms. For Emerset, it is better to be given at the time of the symptoms or right after the onset of symptoms. For most children 2 years of age and older, over the counter, Dramamine is a safe option for preventing motion sickness as well. I do not want to limit his diet during the travel because dehydration can make symptoms worse. You may, however, limit any regular diet if he continues to have symptoms but continuously provides fluids such as Pedialyte. You can also have your child to sip a little bit of ginger containing drinks to calm down vomiting with such as Ginger ale. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr Katie Lee, General & Family Physician