Suggest Treatment For Recurrent Diarrhea

Hello, My 20 yrs. Old nephew is heaving lots of health issue, 1) Anything he eats or drink, he get diarrhea, this is only started when we were in Mexico for one week of Vacation in year 2015.. Now he is continuously loosing his weight and he can t eat anything. In short anything he eats he start diarrhea, I am really really worried, We went to several doctors and checked everything several report have been done on him. So far Negative. Nothing that indicates that his condition can get better. Please can someone recommend a better solution or good experienced doctor who know how to deal and he can be very minutely check everything about his health. My nephew bright future is in danger, he has to dropped out form his college, Please can someone help to find the solution to diagnose his problem. I am very worried about my Nephew.