I think my child might have the flu already. What should I do or give him?

Pediatrician. It sounds like a serious talk with your per. Is in order. Do it for both of you. Ok?
Call the doc. There are simple ways to confirm it & if you are early there are meds that can diminish the intensity of the illness. If it's not really flu those meds won't help. Flu has peaked & started to decline across the country. I haven't confirmed a case in 2 weeks.