What Causes Abdominal Pain, Vomiting And Diarrhea In A Child?

Hello. My son is three years old. For the past 15 days, he has had GI issues which include intermittent severe ABD pain, occasional vomiting, diarrhea alternated with constipation that is yellow and green in color, and lethargy/weakness.... He had had episodes of pain so severe that he is doubled over, and is unconsolable. He has been seen on four occasions, the first of which intusuception was ruled out. WBC looked ok on Sunday, and both CDiff and parasites have been ruled out. I KNOW there is something wrong with my son, and the doc is not listening to my concerns, but is blaming constipation. He was up from 10pm last night and could not get comfortable enough to go back to sleep... he vomited once last night, and had three pretty normal looking BM s, but that did not seem to eleviate the pain. He is now dry heaving. please help... what else could this be?