All you want to do is just get him to associate the barn with work and away from the barn as rest.
This is what my trainer told me. It's getting the right steps so that your horse finally gets it, "Going back to the barn =work. Moving forward= fun/easy/rewarding. Let's move forward!" Easier said than done of course.
I only ride out alone on a solid horse who prefers being alone. That's because I'm still a beginner. I ride very close to the barn where my lessons are (I can always see the barn when I'm riding) so it's not like I'm far away, but anyway....
My trainer said there are 3 kinds of trail horses
1) those that will ride alone regardless of their rider
2) those that will ride alone only if the rider is experienced and confident which makes them feel confident
3) those that are scared to ride alone regardless of their rider.
for these types of horses she said she will go back and forth between the barn and the trail, slowly getting further away from the barn each time. This way it eases the horse into riding alone and they see that going away from the barn is okay.
Another trick is to go out with another rider and slowly work on separating, then coming back together. Keep repeating. The idea is for the horse to slowly build confidence in being alone rather than just make him go cold turkey.
I don't know if this a bad idea or not, but one of the women at the barn said that she would give her horse treats during points on the trail. He loved peppermints so he'd get a peppermint for each 1/4 mile or so. He started to associate the trails with being something yummy and said that she could tell his mind was on getting the treat not on being scared. LOL such a horse way of thinking.