Videos would be easier to work with, however going by the ridden photos, to me it looks like he's stuck in the back and just moving his legs, which explains his being in a 'fixed' position through the neck. He appears to be tight in the neck just in front of the wither. Coincidentally I was just commenting on a thread in the dressage section about this - release the back and unlocking the neck to develop strength over the topline. Have a look in the dressage thread, I think the post is called something like 'building muscle in your horses back'. Certainly worth a read and the posts will help you with this horse.
Once he releases over the back and neck, his paces will improve and you will be surprised at how quickly he'll build topline.
Overall, a bit hard to judge when he's tacked up, however I like the slope of his shoulder, his neck ties into his wither quite well and will improve in appearance with more topline, pasterns appear a little dropped but this may be how he is standing in the last photo. A little straight behind and doesn't look like he will naturally come right under himself in the collected work, however with strength and gymnastic training, this will come in time.