Hi 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.