It seems like your son is suffering an acute attack of viral exanthematous fever
. It may be a simple flu, or even Dengue or other vector borne diseases.
Please get his complete blood count
done along with the platelet count
Treatment is supportive. Give him plenty of fluids to drink and fresh fruits and salads to eat.
Anti-pyretics like Paracetamol
depending on his weight for the next 2 to 3 days and adequate bed rest will help him.
I wish him speedy recovery.