"I decided to do that join up thingy" but you had never heard of Monty Roberts. Maybe you'll become a convert. Monty - a wise old cowboy - believes and preaches that horses have brains - not quite like humans but nevertheless,that horses ain't so dumb as some people like to think. He is very much into getting a horse to do what you want it to do because the horse thinks it is a good idea. It is a philosophy as much as a training method. It is a skill which comes with practice.
Monty's speciality is demonstrating in a round arena in front of an audience that most horses with behavioural problems can be helped and put on the right track.
A 17h1 Trakhener is a powerful animal and mostly they are intelligent.
You might never find out what nasty memories this horse has in its head but probably he won't now ever trust all humans and maybe for good reason. Your job is to get him to trust you - that is what counts.
Horses are motivated by fear, food and hormones. Humans get round issues with horses by using their superior reasoning power and giving the horse security, a sense of discipline and routine. Go back to basics and work him from the ground in the pen - but wear a riding hat and steel capped boots. One of your first jobs to work towards is to get him to follow you at the shoulder on a loose lead rope using only a light touch. Use a gentle voice. Stroke him. Groom him. But please take your time because he may have been wounded by his previous life.
You will have to trust your life up on this big chap, You have got to get inside his head. However at the same time you must not let him be the boss. A tricky thing to balance. If he was an alpha male then when cornered he might turn on you. Don't ever get angry and show it.
As the lady has said: Get Monty's books and videos. Sign up for membership of his group and receive his monthly message. There was another horseman - a man called Tom Dorrance who sadly is dead now. His book "True Unity" is to be bought on Amazon. Difficult to read but it illustrates the thinking behind Monty's work.
Take care, if this fellow doesn't come to trust you, then he will have you off and that might hurt. Best of luck
PS Intelligent horses can be the most difficult to handle but very often, when they come right, they give the biggest rewards.