It can well be a virus.
For his tummy ache, he might need medications like Lansoprazole
15 mg once a day for about 5 days.
Its okay if he atleast drinks well- give him plenty of water, fresh fruit juices, milk, porridge, vegetable soups etc.
Hope he is on medications like Acetaminophen
for his fever
. This will help his muscle aches too that can happen in viral fevers.
Since its intake is poor- he might not pass stools regularly.
Fever in flu
and other viral illnesses usually lasts for 4 to 5 days.
He will be okay soon I guess.
In case if fever persists, he continues to be dull, he develops loose stools, he refuses to drink too, his urinary frequency is less than 4 to 5 times in a day- please do not hesitate to visit your doctor again.
Hope my answer helps you.
Wish your son a speedy recovery.