My Friend Is A Male Aged 18 Who Turned 18

My friend is a male aged 18 who turned 18 in late March this year and is having an over active sex drive. All he ever thinks about most of the times is having sex, it's more of a reflex action than a voluntary one. He has not taken any medication and has not seen a doctor yet, what could be the problem? What could be causing the problem? What should he do?