What Causes Severe Stomach And Chest Pain Followed By Vomiting?

My 5 year old son has had bad stomach and chest pain in goes from below his navel and up to his chest it is followed by vomiting dark that has a bitter smell and the he gets water poops he will do this for a day to a week and it has been on going on and off for over a year but it seems to be getting worse over time. if he does not have food to vomit he will dry heave and it can last for days and weeks.