More Distance & Obstacles
When you get to a distance where the horse is in another pasture and must go through an opening in the fence to get to you, position yourself to help him succeed. Stand at the opening and as he starts moving towards you, back away so he has to pass through the opening to get to you. Next, stand at an angle so he must walk to the opening, then curve to come to you. Continue building this until you can cue him with the halter from any corner of your enclosures.
If you want him to learn to come to you when you are standing on the other side of a fence, practice that separately, adding distance as he is successful.
Add time to how long he wears the halter until he is able to wear it as long as needed. You can do this by putting on the halter at the beginning of a training session, then cuing other behaviors he knows. This will not only add duration to wearing the halter, but also help him learn that he can do other behaviors with it on and he does fun things when it is on. This starts to build a positive association with the halter.
Now, you need to add distractions. Have one other horse in the field with him. Have two etc.
Practice when the wind is blowing, when a person walks by, when a dog is in the field, when there are cars are going by etc.
Practice while he is grazing and see if you can use the halter to cue him away from that.
Carefully control the distraction criteria as much as you can. This process should result in a horse that is eager to come to you when presented with the halter.
Keep it Fun!
Always make sure it is fun. Combine releasing him with other games he enjoys to Premack the behavior. And in the future when you get him to come to you to go do other tasks, periodically clip on the lead, then unclip it, take off the halter and release him. Giving these times with nothing else asked of him is very reinforcing and can prevent the horse that learns that every time the halter is put on, he must go to work. Of course, if you are clicker training him, training is not work, it’s play and he will enjoy doing it with you!
Avoid Poisoning the Halter Cue
If you need to call the horse to you to do a task that your horse does not currently enjoy (attitudes can be changed towards almost everything with the clicker), cue the halter then leave him for a short while to do his own thing, or cue several other known behaviors before doing the disliked task. This will help him to avoid any direct association with the halter and the task.
A common example is haltering the horse, then clipping on the lead rope and attempting to lead the horse into a trailer if the horse does not like it. This guarantees that the next time you cue the halter, the horse will be hesitant in coming, thinking that he may be going into the trailer again. Sometimes you have to be creative, or plan ahead, especially if there is a time crunch for that task to be completed.