Here comes my critique.
When cutting the horse in thirds, he is very well balanced which is great, because one of the most common conformation faults is a horse that is to long in the back-which is not the case here. This gelding is definitely build downhill, but that's something you can definitely work with. I wouldn't be concerned about it.
The length of his neck is the proper length for the size of his body. The length of his neck should be the length of one third of his body, which in is the case. The neck is very under developed and is really lacking top line, but having said that if this horse only does trails it wont be doing any flatwork that would allow him to build that top line. The base of his neck is level with the shoulder which is where you want it to be. A well tied neck is important for good balance.
This gelding has a low stifle. It should be at about the same level as the elbow, which isnt the case, because how downhill he is. He also has a low hock. In this case it would give him more power from behind should you ever decide to take him into dressage or other disciplines. Having a lower set stifle and hocks also means his hamstrings along the back of the hindquarters can be longer and stronger.
I like the legs of this guy. He's big bones which I really like. You want longer humerus to have more leverage therefor having more power. It will also help with the range of motion. A long humerus is always more desirable. Its hard to see the knee. It looks flat on the front with no roundness or bumps. The knee joint itself seems to be large and well support this horse's built. There is lots of space for the flexor tendon as well. It is in line with the canon bone and forearm. More pictures would have allowed us to see this horse from all angles.
The pasterns are slopped properly. It will allow him to withstand hard work on tougher surfaces an rougher terrain which is what you want for what you would like to do. It also shows us that proper pasterns will put less stress on the tendons, making it less likely to get injuries. Pasterns that are to long are usually weak. This horse as expected, is under developed in the hind end. Is this horse sickle hocked? Hard to tell if he is, or if it's how he is standing.
He has a decent shoulder. The slope of the shoulder, should be the same as the slope of the pasterns. It looks like he should have pretty smooth gaits based on that shoulder. Having a good slope to the shoulder, also tells us he will be able to have longer stride. A "normal" shoulder should be between 47-55 degrees.
Realistically any horse can become a trail horse. I think what you need to look at is probably more the temper of the horse than anything, and whether or not trails is enough to keep that horse happy.