Some horses will do this out of discomfort, like then they have too much metal in their mouth. Some horses do it after being fed things like sugar before a ride.
Try to cut out all snacks before you ride and playing with bits to find something he's more comfortable in.
He's in an offset dee snaffle and is very comfortable in it. I know my horse and would be able to tell if he is in discomfort, and he is not. He doesn't get snacks, he won't eat anything but carrots, apples and cheez its. It may be a discomfort thing for other horses, but not for him. I'm not as inexperienced as you may think :wink: