I second the Waterford. But always do due diligence and do your own research.
Try a leather bit or a Waterford but. The less there is he can fight the less he'll try it. There are also both super soft. My horse tends to be pretty hot while jumping or hacking out but when I use these he's guide concentrated on me. And I use them for everything hacking, flatwork, jumping even hunting.
But you could also try to ride for a while without any bit maybe your horse reacts different. If both things don't work let your horse teeth from you vet check. It always can be some medical issue.
Hope that helps...
On my horse I use a rubber single jointed eggbutt snaffle and also in one of my other horses I use a D ring single jointed snaffle with copper rollers I have also heard mylers are good for flatwork too!
Hope this helps
What bit are you currently using?