It think he needs a complete 'overhaul'. I used to get in horses like this and treated them like the spoiled studs I used to get in. [some of these had crippled handlers or at least put them in the hospital.]
I do not like whips to fix this problem if a horse is this aggressive. It only maddens many horses that are this bad. The ones it does not madden learn to fear the whip and not respect the person. Then they attack worse when someone does not have a whip in hand. I have seen it many times and some of the hoses I got in had come from that exact situation. When I re-school a viscous horse, I want them to know 'I' did it and that they are lucky to be alive when I was done with them..
As a side note -- I find it interesting that this horse is a PP flunk-out. Aggressive horses frequently come from people that have tried that program with them and failed. It is a form or 'picking at', 'pecking on' and 'nagging' I hate so much and some horses hate just a bad. MANY sour and ill-tempered horses come from Parelli homes. This horse just took it to the next level. He REALLY hates being pecked at. I would NEVER use a whip on this horse at this point. He may just come and hunt the other end of it or wait until you don't have it one day.
There are 2 or 3 ways to accomplish this. None of these methods are for the novice or faint of heart. But, honestly, I do not know of any other way to get them safe for anyone. They just learn who to intimidate and who to attack otherwise.
One of these methods is to work a horse on a line in a big pen or an arena at the very least. [Some horses are past this method, but probably not this horse.] I have put a lip chain on several and let them work a little to 'feel' them out. [I use a big cotton rope about 12 feet long with a chain braided into the end of it.] Then, I start trying to 'push' them around (mostly making them turn to the right) and make them back up -- a lot. Any time they 'bow up' and refuse to give ground to me -- immediately -- I get after them very hard with the chain shank. By hard, I mean repeated jerks as hard as I can jerk the lead-shank. Many will rear up and strike out. Some will attack. I repeat: This is NOT for the novice horse-person or the timid person. [I once watched husband run backwards and barely get over a fence while trying to jerk the shank hard enough on an attacking mare to stop her. We laugh about it now, but it sure wasn't funny back then.] Most aggressive horses get wide eyes and start running backwards. You just have to be ready for whatever a horse throws at you. Once they start going backwards from you, pursue them and make them back another 100 feet.
Once one backs up fast, you can snap a second lead on their halter and go back to just pushing them around and backing them up in a controlled fashion. The instant a horse hesitates, jerk them hard again (no little bumps with the lip chain) and say a sharp "Ah!" and then go back to lightly asking with the plain rope. If you do this right, you can wean them off of the chain very quickly and get them to listen to just your body language and plain lead-rope. I put them up pretty quickly after I get the initial correct response. Each session can get longer, but I do not linger a lot on ground work. I usually keep it to pushing one away and backing one up for a while.
Once I have a horse backing up lightly, I just go back to handling them like any other horse. I do not handle them roughly, but I do keep a really close eye on them -- not turn your back on one like this person did. If you go back to handling one like a very gentle horse would be handled, they usually are through with it. Some horses, especially studs, I will handle with 2 lead-ropes for quite a while, one with a chain and the other one just a plain lead.
The big thing is to never just 'peck' on one. Try to eat him alive or leave him alone. Pecking and nagging is what got him this way. It will get him right back there.
Once a horse 'gives it up' their eye and their whole demeanor just changes. Actually, the very first aggressive horse I bought that had attacked many people was a TB / Morgan 6 year old, good looking, bay gelding. This was one tough, rank horse. He not only bucked and reared under saddle, he attacked anyone on the ground. He would keep his ears back and grind his teeth when you went to halter him. He was incessantly mad. He would paw or kick at you during grooming and saddling if you took your eye off of him for one instant. I got after him enough to handle him -- sort of. Finally, I decided I had to get through to him and being nice and trying to show him the right way was not getting it. He was going to hurt someone or end up in the killer pen - where I got him. [Back then I was young and idealistic and thought I could save them all.]
When I had had enough and he had almost gotten me 3 or 4 times, I put a chain under his lip and tried to 'kill' him. [Previously, I had worked on the track and thought a lip chain was meanest thing I had ever seen.] I thought mean horses had just not been treated right and they would 'like' me and want to do the right thing if I treated them right. WRONG! Old 'Red' blinked his eyes, ran backwards and truly though I was going to kill him. THAT worked!
After that day, he greeted me with his ears up and went to riding like a horse that wanted to learn. He probably taught me more than I taught him. I started him over fences. He was almost talented. He did not 'tuck' his front feet well enough to ever be good (or safe) over big fences. When I moved and set up my first full-time training operation, I needed to turn Red into $$$. I sold him to a big H/J barn near Parker, Colorado. Ten years later they were still using him for a lesson horse and showing low level hunter and Dressage shows with him. Several times they had called me to see if I had any horses suitable for them. They wanted another one just like him. [If they only had known.]
If this horse is tougher and meaner than that, he needs to be laid down or put in 4-way hobbles. This require even more skill. Restraints work on about all of them. If it doesn't, a 30.06 will.