I’m on the “more beatings will fix this” train. I’m not talking abuse – you should be able to accomplish everything you need to accomplish without ever leaving a mark on your boy. However, if your horse has, in-hand, managed to damage a human being to the level of “probably needs doctoring”, his ground manners are not sufficient to the task and he needs to be fixed.
When he rears: Each and every time he takes the front feet off the ground, you have two or three seconds to convince him that this is the WORST IDEA EVER. Make the most of these opportunities. When he goes up, right away advance on him vigorously (yell, stand up straight, wave arms, look BIG). Your goal is to get him to give ground to you, to back up out of your space, to yield turf. (I’d use a dressage whip in-hand so that a safe distance can be maintained.) Act (but don’t actually be) VERY MAD. You need to be (1) quick about bringing the fury – he goes up, you get after him immediately (2) big and loud when you are correcting (people should stop and stare, like that big and loud) and (3) done in two or three seconds. Once your correction is over, you are going to act like he’s all better and will be a good boy now. So, you give back the slack in the lead rope, try leading him again.
A lot of times, people have trouble opening the door to the good boy box for their horse after a correction because “he reared last time” or whatever but it’s super important to do this. If you don’t open the door to the good boy box, your horse can’t go IN the good boy box. You gotta open the door. (He can’t open the door because he has hooves. Also, you’re the smart one and the grown-up. Open the door for him.) Offer the horse the good deal. Many times when a horse rears up, the handler lets the lead slip through their hands and backs up out his way before going “oh, yeah, that’s bad. I should correct the horse.” So then they correct, the horse comes down, and the handler comes in and holds him right up under his chin tight like as if they’re gonna hold him on the ground that way. In that situation, the horse gets slack and space WHEN HE REARS and when he puts his feet on the ground to be good, the handler hangs on tight under his chin and crowds him. This way of handling the rearing horse makes the right thing (fronts on the ground) uncomfortable and the wrong thing (rearing up) comfortable.
What you want to do is if he rears up, you go after him and really make the rearing up unpleasant for him. You get big and loud and scary and whap on him with your dressage whip so that he backs up away from you and (hopefully) puts his feet back down. (If your horse falls over, trying to scramble out of your way, which he probably will not, let him stand back up and think about things and then lead him off again like he was sane. It is no big deal, don’t treat it like one.) So, you’ve moved him out of your space and whapped him a couple of times and he’s put his feet down on the ground. Rearing and correction is over. What next? You soften your body language and gather him back up, with about a foot and a half of lead rope between YOUR HAND and HIS CHIN. That slack is the open door to the good boy box. You gotta open the door to the good boy box so that he can go in it. And now go to lead him off. If he goes up again, you repeat what you did before, same as you did before. Be consistent. It will likely take a while to fix him if he’s a confirmed rearer.
When he runs over your body or tries to shoulder you out of the way: This is a total lack of respect and a failure to grasp the basics of leading. When he does this, back him up, immediately and with violence. I use knotted rope halters (which most horses respect pretty well) because they don’t overtighten and release immediately when the horse gives, but if flat halter and chain is what you’re comfortable with, use that. (There is nothing magical about knotted rope halters but they are a well-designed tool for working with misbehaving horses.) Back him up a lot, briskly, vigorously, and loudly. (Again, bystanders should think you’re “killing” him but mostly it’s not a contact sport. You use big and loud body language plus a few well timed yanks on the halter to move him out of the way.) Twenty feet is not excessive – during the backing up (which should take three to four seconds) he should be working quite hard to give you ground and not fall over. He should be almost scrambling backward. You really want him to rethink his life choices, here, and he’s not going to do that with a careful and sedate backing up experience. When you’ve backed him up for your couple of seconds, release, be soft, take a deep breath and relax, and continue on. Remember to open the door to the good boy box – he gets about a foot and a half of space on the lead between his chin and your hand. If he’s good, he continues. If he’s bad (takes out the slack by zooming forward), you go back into Boss mode and back him up again with the same Sound&Fury violence (and a few well-timed yanks, but not more than is necessary – a lot of the time you don’t have to yank more than once or twice and then just keep your body and voice loud and scary to keep him moving his feet) you used before. Again, once you’re done correcting, give him back the slack and expect that he’ll be good. It will probably take consistent treatment over a period of time for him to get the idea, here, especially if he’s been allowed to be wrong for a while. Don’t give up.
Good leading manners are a horse that respects the slack – that leads all over, around any obstacle, on a trailer, through an in-hand trail course, at the walk or trot or canter, to halt, all the time with that same drape in the lead. A horse’s leading job to watch you and go where/when you go at the speed you go and stop and wait patiently (without fussing) for you when you stop. EVERY TIME. He should do this for you (the owner) and also for all people who handle him. This is totally a reachable goal and it’s one well worth pursuing. Also, typically improvement in the “on the ground” relationship generates improvement in the “in the saddle” relationship, so there’s another reason it’s worth doing.