Hello My Son Who Is 18 Years Old Has Nausea

Hello my son who is 18 years old has nausea every morning it lasts most of the day then decreases at night. He is 6' 134lbs he lost weight approx 1 1/2 years ago he was 150 lbs. We went to gastroenterologist and he has a endoscopy and colonoscopy which showed nothing. He took omeprazole for 1 month and it did not help.