I usually don't say something this vague, but there's something about his hindquarters . . .I had an odd impression when I openned the thread, before reading any other posts. It is perhaps, that his hind canon angle is not upright, yet his legs are pretty far back. Also, his croup is short and rather flat, almsot as if his pelvis is rotated a bit forward, (meaning the tail part lifts up, the front of the hip tips down).
There is just a strange sort of disjointed feeling between the front of the horse and the rear. I apologize for being so vague, this is an impression only, and the photo doesn't have him truly squared up, so could be way off base. The horse has a nice neck and pretty head and if he moves well and has the brain, then he could be a good buddy for you.