If he is fairly new then it is very common. Many times chameleons will walk around everywhere when they are in a new environment. Also I have noticed that many of my chameleons come down when they are looking for food. But depending on the species of chameleon, I would look into getting a screen cage.
He pobably wants to get out or explore,
but if you see him rubbing his head against the glass as if trying to get out then he could injure himself from constantly rubbing up and down on the glass, it would look like hes rubbed the colour off the tip of his nose scales after a few days of it.