I second what gypsy said. Take him out in hand, or teach him to longrein, and have adventures that way.
Is it possible for you to work him in a field hes not used to? It is a new place to him, like the trails, but is enclosed enough incase something happens. Dont work him all the time in this field, vary if possible. It could be the needed step between the safety of the pen, and the big wide world!
Do you have another person you could ride out with? Another horse may be the encouragement your boy needs to face those trails, and its good banter hacking with someone else :-D
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz