OTTB - I just can't imagine why you took this animal on. For some reason the
poor chap is traumatised. I sat and thought what I might do in your shoes - for what they are worth here are my thoughts:
First I suggest you go out and buy a diary and record every day that you work with this poor animal. One day maybe you'll have the basis for a book.
Undoubtedly you have to go back to square one. You will get nowhere until the horse allows you to fit a head collar and groom him. and by the sound of it, at the moment you can't near him.
So: all domesticated horses need:
food, security, shelter, companionship, freedom from fear & in this case HELP.
I would try to create for him a home pad - say an acre sized field with a field shelter. You might have to divide it up to prevent laminitis for when the grass grows too richly in the Spring. Then find him a companion he will accept maybe a shetland pony or a donkey, perhaps an ageing horse or pony - some creature which can live out with him but which is smaller. Let him choose his companion - experiment. The field shelter is to become his shelter, his feeding centre, his training centre and his safe house. No other horse is to use his paddock or his field shelter, only Your Boy and his companion. The pad is to be 'his' and to hold his smell.
The shelter must be waterproof and wind proof. A very slightly sloping floor is good but cover it with half inch thick rubber matting. Cover the matting with pellets of wood powder. Remove daily all droppings and all saturated wood powder.
The shelter is to be left open but it should be possible to close and secure all gates
so as to make it possible to contain Your Boy from time to time. As routine, let him come and go as he pleases.
Vet. Presumably you have already had the animal vetted? Worm him.
Check his blood, his urine, his faeces. Then check his teeth and his feet.
Whatever you do, do not be present at any 'firm' handling. The tests must somehow be achieved but you personally must not be associated with any aggression. You always appear after the tests with whatever 'treat' you have discovered the animal likes. As he becomes more receptive to 'technicians' then you must always be present when he is examined so as to calm his fears.
Diet. Photo him. Measure him if you can get near enough. Then get in touch with an animal nutritionist at one of the local horse feed suppliers. Your Boy will need vitamins and some minerals ie salt & magnesium. Some vegetable oil and plenty of fresh grass and carefully selected hay. Keep a note of his weight and measurements. Of course water must be fresh and freely available.
Then discover something he loves to eat - apples?carrots?parsnips?pears?sugar?
Wearing the same coat (never to be washed) and the same hat visit him every day - morning and night. Split his daily ration of food into two and personally always give it to him in the same bowl, in the same place and as close as possible, at the same time. Talk to him constantly. Call him by name. Recite to him a story.
Every time you visit, offer him a carrot - or whatever it is you find he likes. Make him addicted to it. First offer the fruit on the ground, then a plate and finally by hand. You'll know when he is addicted, he'll come and nudge you to ask for his 'treat'.
Be advised he recognises humans by smell, voice, body language, walk, touch and finally by sight. He should come to recognise your footfall or the engine of your car. Maybe one day, if you are winning, he will whinnie upon your arrival.
Treat this creature as terrified - panic stricken. You have to get him to trust you to the extent that he will allow you to stroke him and groom him. You do that by making him feel secure and 'loved' through the routine of supplying food and regular constant attention - even if at first you are rejected. At all times you are to be quiet, firm and persistent. You never ever raise your voice, wave your arms about or get angry. So wear boots and pads in case the little devil catches you out.
Try taking a book and a chair into his paddock and just sit there reading.
Until this horse trusts you enough to allow you to touch him freely - then other schooling is wasted. Your aim is for Your Boy, whilst wearing a halter, to come to walk calmly at your side on a loose lead line. If you can get him to trust you that much then you have a chance. Remember one day you'll have to put a bit in his mouth and lift all four feet to clean out the frog.
You will have to commit yourself to a long haul. Are you sure he is worth it?
How much patience have you got?