"The problem is, as soon as you get him on the lunge line and try to get him moving, he kicks at your head. I've even had him do this for a professional trainer. It's not even him kicking just to kick. He turns his butt at you and will back up in your direction. He's even chased me once or twice. He managed to get me in the chin once. It dislocated both sides of my jaw and split my lip."
"He reacts to this in three seperate ways. The most severe way is, instead of turning his head towards her when she pulls it in that direction, he well violently buck backwards. Basically, she's pulling his head at her, and instead of turning he's just bucking into her.
The second reaction that he has done in the past is pretty much the same thing, except he will bunny hop backwards instead of violently buck, and try to knock the person down with his butt. He has only done this with the first trainer, though, who doesn't see him anymore."
"He has other problems, too, but they're easily explainable. He's horrible with the farrier, but at the same time, he's young, doesn't understand, and gets bored easily. He's fine if he's busy (like if I pez feed him treats). His previous farriers just sedated him, which wasn't solving the problem. I found a great farrier who will hold on to him even if he has all four feet in the air or has laid down and is trying to roll over (he seems to think that if you pick his foot up high, he needs to lay down).
The second problem he has is when you lead him down a hill, he charges. I'm not sure if it's a balance problem, because it's only on steep hills, or if he just thinks it's fun. A place I had him boarded at had a steep hill, and they told me when the sun started coming up he'd run up the hill and run back down, for apparently no reason, until he was fed.
Other than that, he's perfect..."
This is not a horse that merits Internet analysis. It is a dangerous horse. You couldn't sell him if you disclosed his behavior, and you'd be liable if you sold him without disclosing. Even with full disclosure, you probably have some liability risk if either the trainer or farrier is badly hurt.
If I owned him, the answer would be a 30-06. Why? Well, I recently sold a purebred Arabian mare, 8 years old, for $600. She is small, but could carry 200+ lbs on her back all day. We bought her for $800, I spent around $2000 having her trained, then rode her until she was more experienced, and then my youngest daughter started taking lessons on her. If she didn't hate my gelding, I'd have kept her even tho we didn't need 3 horses.
She is spirited and enthusiastic. She expects to be treated like a lady and gets bitchy if she thinks she isn't - but she'll give you everything if you just say please and thank you. She loves people, loves to just stand near them, stands quietly while being groomed, tacked up and mounted, lifts her feet for the farrier and was a sweetheart getting her teeth worked on. She can be lunged free or on a rein if you wish, or you can just get on her and ask her for what you want.
Her market value on the current market was probably around $1000-1200, but I didn't advertise her. A couple living near me wanted a younger Arabian so their 27 year old Arabian could retire from trail rides, and she now lives 1.5 miles from me.
And I sold her for $600.
What is the point of keeping a horse that has broken your jaw and acts aggressive around humans? You can make your own decisions, but if anyone gets hurt by your horse, they may think you should be held accountable for those decisions. I don't know you, and you can take any chances you want.
You wrote "...he's definitely a loved member of this family." That's part of the problem. Horses are not family. They are not human. But if my son broke my jaw, or attacked others, I'd put him in prison. The time for understanding would be past, and it would be WAY past the time for asking advice on the Internet.