What Causes Chronic Cough,headache And Constant Diarrhea?

Afternoon,
My husband is 45 years old, aprox 205 lbs 5'11. For almost 20 years, he has been battling multiple symptoms for many years. Nausea almost after every meal he eats, a bit of acid refulx, chronic cough, chronic headache's ususally at the base of the neck and top of his head and constant diarhea. Bit of background history is: He was born premature 2 1/2 months early. Was in Childrens Hospital for one month until he was able to go home with his mother. He never had any serious health issues growing up besides a common cold and flu, and a sprain and broken arm around 10 yrs old. Had a benign tumor in his back and was removed 15 years ago. and Had his tonsils and adnoids taken out around 6 years ago. His symptoms started around 20 years old. Has taken multiple allergy and pulmonary tests all with negative results no allergys, no asthma. He had 2 colonoscopy performed, In August 2012 he had a couple of pulpits that was removed, and had another colonoscopy last month all with a clear result. The dr performed biopsy of the stomach linning and all with negative results with the exception of a little acid reflux. Because of the nausea and chronic cough he ate cough drops over the years and developed multiple cavities and tooth decay which now has caused an infection to his gum and front tooth. Any suggestions or insight will be greatly appreciated.