You could pony him or go for walks with him. He'll get to know a lot of things if you do that. You can also take him for short trails (e.g. 20 minutes) and just lengthen the duration of your rides. In my opinion 2 1/2 years is too young to work a horse hard (they are not really grown up then), so give him time.
I usually do a lot of sacking work before I ride my horse. I will lunge them with bags on the saddle. If I have someone to ride with I will take my horse out on a trail their 2nd or 3rd ride and see how they do. Every time I've done this it worked out great. Keep it to a walk/trot the first little bit. Don't rush cantering as you don't have as much control when they are first starting and if they spook they could take off on you.
Ponying is a great way. I used to Pony Hunter a lot when I was at my old barn, can't now with no horse to ride. I am going to trailer him over to the park and hand walk him. I would make sure you have a good solid walk/trot with him before heading out to trails. I have sat on Hunter but am not going to ride him until the fall when he is 3 (wanna let him grow a bit more)
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.
I had an awesome percheron I used for ponying. It really did good for Hunter. Cars, trucks, tractors, buses, motorcycles you name it don't phase him. He has crossed a lot of bridges and water following his trusty buddy Duke. That will be a huge help when I actually ride him out there.
If I can get the horse to lope with the saddle on, from the ground, up and down little hills or over a pole without being too bucky then I get on. Once on if I can turn left and right, move his hip around without to much bracing, stop and back up I just head for the trails and ride it out :lol:.