My 15 Year Old Son Has Had Low Grade Fever,

My 15 year old son has had low grade fever, headache and neck pain for 6 days. Was diagnosed with ear infection on day 3. Went back to the doctor on day 4 because of nausea and fever rose to 101.5. Went to ER and was given an ultrasound on neck as well as blood work. They found white cells normal, mono spot test negative and the ultrasound showed enlarged lymph nodes on one side. The fever is now gone but the lymph nodes are larger and more painful. Ear infection is cleared. Any ideas?