You may know him as Alan Harper in TV's Two and a Half Men.
On the show he has a full head of thick black hair when actually he's as Steven Seagal.
He started show early signs of balding in the early to mid 2000 as seen in this photo.
I guess the studio thought that Jon's balding head didn't seem fit enough to react well with audiences so they invested in a wig of some sort so no one in the audience would realize his baldness but Jon has been seen loud and proud walking around town.
He really should get a hair transplant as he doesn't suit being bald as people are so used to him with hair, just ditch the toupe and buy a hair piece.