What Causes Hematemesis In A 2 Year Old And On Omeprazole And Domperidone?

My two year old son has had melena seven times in his life. Sometimes this is accompanied by hematemesis (sometimes dark brown, sometimes bright red). He has been on omeprazole and domperidone since 3 months and every time we wean him off, we have an episode. He has recently had a negative PH reflux test and therefore come off the meds, he has also tested negative for blood clotting disorder and coeliac (he has had up to seven diarrhea type stools daily for the last six months). Today, 12 days after stopping his meds, he had an episode of melena, and the GP has put him back on omeprazole. He s also had a negative test for varices.