How Can nausea And Diarrhea Along With Stomach Pain Be Treated?

My Son is away at college and has been ill for a month now, his symptoms are nausea , stomach cramping, and little diarrhea. Sometimes a headache but generally just feeling yuck. Initially I told him it sounded like acid reflux so he started taking Pepcid and drinking Mylanta every day and it seemed to get better for a few days but returned with him even worse. He went to the campus health clinic and they said it was gastroenteritis and gave him some anti nausea med and said he would feel better in a few days so when he did not feel better he drove off campus to American Family Care-blood work was normal and they said the same thing- gastroenteritis was going around - they had seen several cases. How long does this usually last ? its been month now since this initially started??