Worried ftm! Baby has the stomach flu

Hey ladies, hoping for some advise. My little guy has the stomach flu. Vomiting finally stopped but still has diarrhea and a fever. I brought him into the children’s hospital because at some point his fever was 40.9celcius. It goes down to 38 when I give him Tylenol and Advil but soon as it wears off it climbs right back up. How long does a fever last in babies. I expected the diarrhea to last a few days but I thought the fever would have disappeared by now but we are going on 48 hours. So I guess my question is how long do fevers typically last in babies and does anyone have any tricks to help them out during that uncomfortable phase in between medicines.

Thanks worried ftm over here!