My horse plays with his bit...?

My TB is currently in a D-ring snaffle bit. He loves to play with his bit..He mostly just chomps on it, nothing real serious. He has done it his whole life (hes only 6) even on a loose rein he does it. I can ride around on the buckle and he's just chomping away. He doesn't throw his head about or anything like that but its kinda embarrassing in the show ring. All you hear is my horse chewing on his bit, haha! It doesn't cause any problems whatsoever and he has his teeth checked regularly. I was wondering what causes this and what I could do to fix it? Is it true that copper bits taste sweet to horses? and if I got a D-ring snaffle with rollers would it help? Thanks!