Does he bolt only when he sees the halter or does he run as soon as a person tries to approach him at all? Is he alone in the field? Can you get him into a smaller field?
If it's just the halter, try a neck collar instead as a step to getting used to something on him. Move to a war bridle slowly by getting him used to a rope on his head.
If he is scared of people, then never mind the halter for now. He has to lose that initial fear first or every other training step will be pointless and dangerous. Does he get any feed on a regular basis? If so, feeding time is a great training time. Be near him. But never let down your guard. He is an unknown so don't let him disrespect your space. As he learns that to associate the food with you/people while at also knowing that he doesn't have control over it you will progressively be able to be near him.
If he doesn't get regular daily feed of any kind, start doing it. Just a simple 12% pressed ration, nothing sweet will do fine. Initially, you'll have to teach him that it's yummy. This will only work if other horses aren't with him. Get his attention and put it down in the field in a bright red bucket. Doesn't have to be much. Just a cup will do. It's a training exercise, not diet change. It's important to do this at the same time every day initially. Back away and leave him be. Do this until he starts to look for you to bring the bucket. Once he starts to look for the bucket, don't leave. Hang out -- do chores, walk around, read a book, whatever. Just be in his sight. Keep progressing to get closer to his space.
If you have time, you could do this exercise twice a day. Just remember, never let down your guard and if he ever threatens you, give him heck. Level of threat determines level of heck.
You should be able to get to the point where you hold the bucket and he eats from it. Move to contact, etc. etc.
Its so tough when you don't know what kind of personality he has or what to expect. If he has been in the field for a year with no care, he more than likely has some serious hoof care needs and who knows if there is anything else going on (cuts, worms...). Poor guy. Any pictures?