What Causes Dry Heaves Along With Vomiting?

My child ate dinner at 6pm which consisted of some fresh cooked oysters. Then he exercised an hour later. He came home and slept at around 11pm. He is 13 yrs old. Hes been dry heaving and vomiting for 2 hours now. His vomit was very thick. So i told him to drink some water...which help ease his vomiting. Vomit has now turned dark brown. What should i do?