#1, teeth floated/dental work as needed.
#2, he might do better with a different bit: roller, copper inlay for good taste & salivation, etc. Make sure that no part of bit is pinching/poking him.
#3, his training must be such that he IS accepting the bit, IS not trying to evade it.
If these 3 are good, yet he still tends to play with it, just use a flash/mouth-shutter for shows, because you reduce the communication with him & are rather riding his head, instead of communicating with him via a flexible jaw, when you strap his jaw to the rest of his head.