Looks like your son is having a virus.
, muscle aches- all are well seen in many viral fevers.
You might visit your Pediatrician
and get things confirmed.
For fever and pain- he might need Acetaminophen
in correct dosage every 6 hours. He will feel better after that. If pain in calf muscles is severe, you may also give him 1 or 2 doses of Ibuprofen
Give him plenty of fluids, bed rest
and heat compresses on his legs.
For vomiting- he might need medications like Domperidone/ Ondensteron in correct dosage according to his weight.
Repeated vomiting is not good- as it will make him feel tired and can result in electrolyte loss too.
Hope my answer helps you.
Wish your son a speedy recovery.