What Causes Low Grade Fever,fatigue And Body Pain In Children?

Hello,For 9 weeks now my son has run a low grade fever, anywhere from 99.4-100.5. He looks pale and flushed cheeks, tires very easily, and complains of achiness. Been to dr 5 times. Last bloodwork showed elevated WBC (14.2) and high neutrophils (69) and low lymphocytes (20). He has tested negative for flu, strep, mono, etc. We are bding advised to now seek a specialist. My heart hurts becuase he usually is so active and happy, but is not now. Has missed a lot of school, because they call me to come get him whenever he runs a fever above 99.5. What could this possibly be?