I personally wouldn't ride him out yet. Start on the ground. Get him used to walking over tarps, around poles/barrels. Find anything "weird" and "scary" that you possibly can. Plastic bags, anything that makes noises similar to what he could encounter on trails. You can't completely duplicate a trail, but you can get him used to trusting you when something new and scary is encountered.
Once he's mastered these things on the ground, do them under saddle. The tarp is the big one. I used this to get a horse to go over a bridge, i couldn't duplicate the bridge obviously. This horse would see the bridge from feet away and wig out like it was going to come eat her, getting her to step a foot on it was hard enough. The tarp is something weird under their feet, if they trust you to walk over it, they'll trust you to walk over other things that they don't feel completely comfortable with. It worked for me.
Once you've done all that, you can hand walk him or pony him out. I've personally never ponied a horse because i don't have any level headed horses to use, but it's a great idea and a wonderful concept.
"Animals are not our whole lives, but they make our lives whole."