My 23 Yr Old Son Has A Bad Tummy Bug

My 23 yr old son has a bad tummy bug for about 10 mths. Mainly abdominal pain an d gastric, can't drink beer or any alcohol without getting sick. He has had gastroscope . Colonoscopy and ultrasound all ok. But he went to Malaysia with the army last November and drank from a river? Dr had advised to take Inner Health to fix his gut. I said to him keep s journal of everything he eats and when he needs to go to the toilet. But he is over it and is affecting his everyday life. What can we do? Please any advice I feel helpless .