Hi It might be because of gastritis. Try to avoid spicy food/ citrus fruits/ coke etc. He needs a course of acid suppressants like Lansoprazole 15 mg once a day for 5 days. This medication has to be given half an hr before food. If vomiting persists, he might need medications like Domperidone/ Ondensetron. Hope my answer helps. Wish your son good health.