fever question

On friday my son started with a runny nose and that progressed until yesterday when he woke up with a fever of 101-102. It was going down with advil until now. I gave him some this morning at it went down to 98.9 but when it wore off it went back up to 102 and it's been 2 hours since I gave him advil and it's still 101. So, he's never had the flu before (he's 4) and he's tired but been eating ok and wants to play but I've been making him take it easy and watch TV. So I'm not too worried, but why is his fever not going down all the sudden?