Thank you for the information,
There are some common causes of such neck stiffness.The common ones are arthritis, injury, meninfitis, subarachnoid haemorrhage, tetanus, cerebral haemorrhage, viral illness etc.
The symptoms of fever ,chill, headache and neck stiffness is commonly associated with viral illness and may cause the neck stiffness in him.It should be ruled out first since he also had fever, chills, and headache.
As per the symptoms the cervical strain may also cause such neck stiffness though the chances are less.
The chances of viral meningitis may be there and should be ruled out by having the CSF examination done after lumbar puncture. The arthritis is also commonly associated with such neck stiffness and should be ruled out.
You should have the relevant investigations for him like ct scan neck and brain , blood counts, lumbar puncture etc. after consulting your doctor.
The treatment of viral meningitis is symptomatic and may require hospitalisation if the symptoms are severe.The other causes should also be ruled out.
Here is the link for more information
causes of neck stiffness
symptoms of meningitis
Let me know if you have any doubt.