i would definatly go see another doctor get a second opionion your doc might have missed something that another doc might pick up....
get checked for parasites... this sounds like it could be a parasitic infection, especially after visiting a country like Peru and eating/drinking there.
Worm infection or some ameoba? Chlorea?
He hadn't been on any antibiotics recently has he? It does sound to me like he has had a food bourne illness. Keep hydrated and do not use immodium as it could make it worse or delay recovery. See a gastroenterologist if it continues as they may want to do a colonoscopy.
Good luck
Paula
Perhaps have him see an infectious disease specialist - he may have picked up something unusual from Peru.