Awww, he is super cute!!
I think you look pretty good overall, I like that you're not extremely folded or ducking with your upper body, and you are using a short crest release which is perfectly appropriate for the jumps you are doing. If I want to be picky I'd say you could push your hands just a TAD more forward when you release instead of planting them near his wither, just to give him that little bit extra rein so he can stretch his neck out for balance.
Your lower leg looks pretty good in most of the photos, but in the rock jump, you are pinching with your knee which has pivoted your upper body forward and your leg back, putting you in a bit of a precarious position. Did he maybe take an extra stride or you were ahead of him with your upper body on takeoff? Awkward jumps happen sometimes, you're not doing it in the other photos so it looks like it's just a "sometimes" thing, but I would just try to always remember to sit tall, shoulders back, weight deep through your heels, and almost think of pushing them down and forward as you approach and go over each jump to keep that base of support right under you. That way, if something does happen, he chips in or stumbles or something, you are good and solid and not as likely to get unseated. When in doubt, always wait with your upper body, it's always better to be a little behind the motion, instead of out in front where you're going to get turfed:P
You mentioned he "slide stops", I'm assuming you mean he refuses every so often and just comes to a stop in front of the jumps? There's many reasons this could be happening, he may not be up in front of your leg and on your aids, or you may be going a bit fast and not giving him enough support on the approach. You didn't say how experienced you or he is cross country, if he's still fairly green he may just be distracted or a little unsure about jumps that are new or look a little different, so he needs a little bit more support while he figures things out. For anything new, I would make sure I trot it first before cantering, a nice bouncy trot with him up in front of your leg making sure he is moving forward but not rushing, and sit nice and tall, shoulders back, leg on, hands up, and ready to cluck or kick and urge him on if he hesitates in the slightest. If you are in any other position approaching the jump, it's going to be virtually impossible to "correct" anything if he hesitates or tries to run out. You as a rider want to be confident and supporting him every step of the way so he understands his job is to go forward and over, regardless of what is in front of him. Once he starts learning what to do, you will learn to be more effective with your body and be able to relax a little bit on the approach, but still keeping those driving aids ready in case you need them. You'll find as you get more experience and get your position more solid overall, the stopping will happen less often because you will be riding more effectively.
If you are both newer to cross country, it will be a bit of learning curve, because you're figuring yourself out and teaching the horse at the same time. Just make sure everything is very progressive and step by step, keep it simple, ALWAYS positive, lots of praise, and don't overface the horse or yourself. 100 baby steps, even though it seems so slow, is so much better than rushing things, becaus then if a problem crops up, it's WAY harder to fix. Ideally you want to take it slow enough that you never run into an issue. You didn't mention if you were out schooling with a trainer or not, but it so incredibly helpful for cross country, I would recommend always going out with a trainer if you aren't already.
Anyways, that ended being pretty long, sorry! I hope it helps though:)