My son is 20 years old, is exhausted and dizzy. his wbc is low 3.9, neutrophils are low at 44.7 and monocytes are high at 17.9. what could this be?

To see GP: He needs to see his gp for further evaluation. Many things can cause low white blood cell count and elevated monocytes count including viral infection like infectious mononucleosis, etc. However other thing like certain blood disorder including leukemia is also something to be ruled out. His gp will examine him and if the blood count looks suspicious-referral to a hematologist can be done.