Motion sickness & naucea. Pls sugges!

My child is 5 yrs 8 months old. he have a peculier problems. When we move with a taxi or private car or bus he vomit. just after he climb in the vehicle we can imagine he feel uneasy after 5 to 7 minutes later he lay down and we feel some problem in his body specialy in stomach. he vomit but after that he just feel tired. But whenever we go by rickshaw this problem is not appear. Sometimes with ac bus this problem not appear, sometimes in ac bus the same problem appear. Sometimes when he feel uncomfortable we change vehicle on the journey he feel comfortable in auto or rickshaw. But sometime on the way to school in the morning he vomit. Sometimes when go to park usually not vomit. But if we eat icecreams or light something he can vomit on the way to returning by rickshaw. His doctor , we talk to him regularly according to him my son have motion sickness and vomiting problem. But I think / in my view he has some chronic acidic stomac or some gastritis or some stomach problem. Pls sugges