Dear Ms Johnson, I am sorry to hear that your fianc is not feeling well. From what you have described it seems that he has lost quite a bit of weight over the last 6 months.
Chronic diarrhea can be due to a large number of reasons.
It is necessary to exclude structural as well as functional abnormalities of small intestine, colon, pancreas, liver and bile ducts. The weight loss and fatty food intolerance suggest the possibility of malabsorption. He reponse to antibiotics raises the possibility of bacterial overgrowth. In this syndrome due to anatomical or digestive derangement food collects in portions of the bowel and is acted upon by bacteria to produce substances that cause diarrhea. Unfortunately with the information at hand I cannot give you a definite answer as to what is causing him to have bacterial overgrowth if indeed that is the problem. .