What Causes Persistent Nausea And Vomiting Along With Fatigue And Hair Loss?

My son is 18 years old and lives with nausea and vomiting on a daily basis. He has been through every gastro test that we know of and has had pretty extensive work up and the doctors have not been successful at finding any cause. He started with the bouts of vomiting at a very young age, we have visited numerous Gastroenterologist as well as several primary doctors. He has had some brief periods of doing okay but it is usually short lived. For the past 9 months he has been declining. We had found that his celiac artery was compressed at the end of last year and the surgeon was hopeful that it was possibly causing some of the symptoms, unfortunately, after the surgery he did not get any relief and in fact has gotten worse. Last August I noticed his hair had started falling out and we went to the determotologist and he said it was just male pattern baldness (my son was 17 at time). I am concerned the hair loss, along with his constant fatigue and nausea and vomiting is all related. All of his blood work comes back normal except his billirubin but doctors say that elevated billirubin alone is only an indicator of Gilberts Disease. In the past month he has now started having loose bowels in addition to the nausea. He is 6ft 3 inches tall now weighing 156 lbs. Do you have suggestions as to what type of doctor for him to see next. The gastroenterologist and the surgeon said there is nothing else they can do and his primary care doctor says he is at a loss.