Wet saddle blankets - lots of riding, if you have to ride him for an hour in the arena, trotting, changing directions frequently and such until he's a bit tired, before you go out away from the arena area, that will help. I have had barn sour horses (which is what this sounds like more than anything else) and it's helpful to ride them out at a trot, and you have to pay real close attention because you need to switch directions and head back BEFORE the horse decides it's too far and too scary, then just ride them in a circle or something a bit, and head back away a little bit farther and again, turn to home before the horse gets freaked. That would be the end of a session. Rinse and repeat. Don't be patting on them and praising them EXCEPT when you are heading away and even then I'd keep that to a minimum. It's real easy to accidently praise them for the wrong behavior. I haven't seen ground work and tons of arena work doing a whole lot for a horse such as you describe; what works it to get out there with a balanced plan.