Based on details your son has fever, headache, neck pain
and tiredness.This can be simple viral fever
with myalgia and tiredness.
He should be on antipyretics ( paracetamol
as per body weight),and give him plenty of fluids. Consult paediatrician for clinical examination.
If fever, neck pain and headache worsens need further evaluation like CT scan brain and lumbar puncture
for CSF study to rule out brain and meningeal infection.