If you dissect him in thirds, he does appear to be a hair long in the back HOWEVER he is quiet young, and there is still a possibility he will fill into his chest balancing that out. If he stays longer in the back, he may be weaker and it will be more difficult to round up. He appears to be a little bit downhill as well, but again not something you can confirm until he is at least 4-5 and he's likely to be nearly done maturing.
The neck is measured from the poll to withers, the neck of a riding horse should be proportional to the rest of his body. His neck is of good length which is great for effective balance under saddle and in his movement, which also allows him to counterbalance the rest of his body. Good neck length will also allow him to shift his centre of gravity making him more balanced then a horse with a short neck. A shorter neck would have made him less supple. It also allows him to flex at the poll and give to the bit. It will also allow him to be more flexible. I think he lacks muscle but that's normal for a baby of his age that has not worked under saddle yet, and with his big build I have no doubt he is going to be able to build a nice topline. The base of his neck is also level with the point of his shoulder which is what you want for good conformation.
I am in love with his shoulder. The shoulder is the best indication about the smoothness of his gait, which is a big factor in the degree of concussion his legs will take. His well sloped shoulders act as a shock absorber decreasing the amount of concussion through his body. He is also less likely to suffer breakdown and lameness due to excessive concussion. The shoulder from the peak of the withers to the point of the shoulder, should be about as long as the heck from the poll to the front of the wither.
Chest-no photos from the front facing back to critique.
His front end looks with a really long humerus which gives him more leverage and consequently more power. He also has a great shoulder which will give him the ability to really reach forward and open up his stride. Again no photos from the front so can't critique on forearm muscle structure which would help guess what his movement will be like. That muscular structure is important to help determine his aptitude for lateral action such as turning quickly, spinning around his hind leg dodging things like cows or side to side action-which for this guy would be important because he will be doing some reining.
His knee looks great. I don't see a bucked or calf knee. The alignment is where it should be. Having a good aligned knee means there will be less stress on his tendons. He is also less likely to suffer from strains. A horse who is under at the knee would be more likely to suffer from carpal fractures as well. Unable to see if he is bow legged because there are no pictures available from the front. Hard to tell by the photo I see, but it looks like he might be tied at the knee. Unclear to me tho because his outside leg is right on the edge blurring the photo a little bit.
The pastern influences the soundness of the joints above it, but when I look at it I really like the length and angle of it. The slope of the pastern should be 45-55 degrees which he definitely meets. With a good slope of the pastern it transfers weight to the tendons and their attached muscles adequately. A pastern to upright would concuss directly into the tendons. With an excessive slope, its putting too much pressure on the sesamoid bones on the back of the fetlock joint and the navicular bone inside the foot.
Hi stifle is set lower which goes hand in hand with relatively low hocks, which gives more power to the hind leg than do a high stifle and high hock. Can't say much more about the stifle as no pictures were provide from the back facing forward. His hock is level with the chestnuts in his front legs which is where you want it to be, again a sign of a well balanced horse. I don't see any protrusions at the front of the hock. He has strong conformation structurally because the point of the hock is directly under the point of the buttock.