What Causes Persistent Fever Despite Taking Advil And Amoxicilin ?

Our 5 yr son presented with fever Sun am. Started Advil/Tylenol.Took to Dr. Mon am. Sent us away with nothing. Mon night had fever 103 most of night despite meds & cold cloth. Persistent fever all week, congested nose but also very runny, very wet sounding cough. Took back to Dr. Yesturday got amoxicilin 11:10 am yesturday, started it. Even with meds fever always returns when meds wear off, no appetite, diarrhea tonight. Doesn t seem to be getting any better, what should we do?