Hello Most common cause of high grade fever in this age is some viral infection. During first few days fever may be the only symptom and other symptoms like cough,cold,loose motion,vomiting etc develop later on. The main aim of management here is to give antipyratic like paracetamol and or ibuprofen along with tepid sponging for fever. Giving plenty of fluids helps in maintaining proper hydration and also in controlling the temperature. I don t know on what basis your doctor has diagnosed him as a case of stomach flu as there is not any suggestive symptom. Also I don t recommend Azithromycin without knowing the type and site of infection. Here I would like to continue Advil and tepid sponging with proper monitoring of temperature. I request you to consult your pediatrician if you notice any other significant symptom. Regards