My son has influenza, fever for 4 days,5th day there was no fever , 6th day his temp went slightly up 37.9 dr gave me antibiotic, not sure to give it?

Flu: if itvis only flu, antibiotics are not needed. If there is superimposed bacterial infection on top of flu, antibiotics are given

Call the doctor back: An antibiotic will Do Nothing to treat a viral infection such as the flu. I would call the doctor back an specifically ask them why they prescribed an antibiotic. Unless they have a clear concern about a secondary bacterial infection or believe it is not the flu there is no reason for an antibiotic and over use of antibiotics can be dangerous.