What Causes Chronic Nausea And Vomiting?

My 14 year old son is nauseated. He has vomited 3 notes in a row about 2-3 hours after dinner. Although he did have dessert later. He came home early from school today because he was so nauseated. We went to the doctor yesterday who thought he might have a virus or migraine. i think he would feel better but he won't eat because he is afraid he will throw up again. What can I give him to eat or drink