This is an arena I've only ridden him in very lightly, with no pressure to perform, no one watching us. I don't lunge or train him in the arena, and there is no equipment sitting out. I've done it some bareback and some with saddle, and he reacts the same way.
He does have a history of ulcers and had been pinfired in the past, leading me to believe he was competed rather hard. Could he have some form of performance anxiety? How do I help him with it?