Very cute horse! He uses his body very well over the jumps, although he could sharpen up his front end. I expect that will come with time though, if he's green.
I think some of the difficulty you're having holding your body over the jumps that causes you to sit down just a little too early, stems from your throwing your body at him a little on take off. I think if you focus on just holding your position, all the way from take off through landing, you won't end up sitting down too early and your lower leg won't want to slide back on take off. Different people are helped by visualizing different things to achieve this - some really benefit from being told to "hold their shoulders" at the base of the jump, which allows them to say in a solid position, while letting the horse's jump close their hip angle, and others tend to do better being told to "wait for the horse's hind legs" to jump.
Overall, you have a nice position and a well executed crest-release, moving towards an auto release at times.
You mentioned that your horse pulled several rails the last time you rode him. This may be from riding to a deep distance - he looks like he wants to jump over his shoulder a little, and that form, coupled with a deep distance often results in rails. If he's green and still figuring out how to use all his parts, adding a ground line at the base of the jump may really help him. Low, ramp-y oxers can help sharpen up a horse's front end as well, and may be beneficial for him. His tendency to want to jump over his shoulder though is another good reason to work on holding your upper body at the base of the jump, to encourage him to rock back over his hocks and jump up.