New problem- bucking/rearing barn sour

First off. If he is alone that's not very good for them mentally. Can you find a pasture mate for him? A goat, a mini, a companion horse? i'm not condoning his actions by any means but a horse that's alone has a lot of mental stuff going on and he might just have huge separation anxiety problems. When it's a mental problem like that you're not gonna get a whole lot of training done to improve his attitude.

That being said, maybe he's just being a *****. Does he buck then when you pull him up he rears? Or does he rear and then start bucking/hopping around? A horse that bucks and then rears goes from GO to WHOA too fast and has nowhere to go with that frustration but up. So when you feel him start to tense you need to move his feet and keep his head up. If his head is up he can't buck and if his feet are moving he can't rear. So start doing the "drunk walk" or "snake"...many terms for the same principle. Move him a little to the left then a little to the right, or in some cases do a circle to the left and a circle to the right. Don't do small circles, because that slows him down and puts him back on his haunches making it easy for him to rear. Do 10-15 meter circles or if it's on a narrow trail, turn him almost sideways to the trail, then go almost sideways the other way but keep his feet moving. If he tries to bolt, don't just clamp. Use the catch and release method, sharp tug then release for a second, sharper tug, then release or tug one rein, release, tug the other. You want to get him unbalanced and then he will slow down. Best case scenario is to make him go in a huge circle, but like I said i don't know if you're in a small trail or riding through fields. Basically you don't want to completely stop his feet, that lets him buck/rear and all the bad stuff. Keep him moving and occupied but on your terms at the pace you want.

More arena and in hand work at home will help and be sure to add lots of emotional stimuli. Speed, changes, spook work, new lessons, anything that gets your horse a bit upset then bring him down again. A horse that is soft and giving cannot be bucking and rearing.

Take shorter rides and try to just get him to the starting point of being upset, then work the exercises you did in the arena until he is quiet and then leave him alone. Get down if you need to be safe, but work him until he is quiet.

I correct rearing and bucking with the same pre-taught cue, disengaging the hip. Get that cue solid in the arena meaning he does it softly from all gaits. Also, work on shoulders in the arena. The more body parts he is soft and responsive to at home under pressure, the more control you will have away from home.

Go on working trail rides. It is mind draining, but initiate all moves, go around bushes, rocks, whatever. Do circles, serpentine, yield shoulders, hips, stop, back up. Keep him thinking about you and staying soft. As he gets better, let him alone when he is quiet, work him when you see him think about acting up.

Trail riding can suck a lot of training out of your horse because we like to drop the reins and enjoy the ride. The longer you go without asking your horse to do something, even if it is to just softening his jaw, the more time he is spending doing what he wants. This is not to imply you should have constant contact on the bit. Every request should end with softness from the horse and a release of the rein from the rider. The better your horse becomes, the smaller the request can get, but we should always check it now and again to be sure we are on the same ride. : )