Why does my 9 year old's head sweat so bad?

That's a great question , my son was like that too , sorry I never found out why or how , hes 12 (13 in March) and he doesn't sweat like that anymore can't remember when it stopped after a while it was normal to me, I also shaved his head thinking it may help but it still got soaked , (he was never a bed wetter though),maybe some kids are just like that .