Tuck back those shoulders! I like to see riding with a loose rein, just bring your shoulders back a bit. You got plenty of rein there, no excuses ;)
I would also check your stirrups to make sure that there not too long, your heels should be down with your toes straight (otherwise, you may give the horse mixed signals).
Squeeze, squeeze, squeeze and try to keep those lower legs steady. Especially when going at a trot or canter. You'll feel more balanced. When in a canter, try to squeeze with your legs and lift up your seat so you only have light contact. Don't tilt backwards, you'll unbalance yourself. You want to be forward and moving with the horse with your legs and seat.
Are you asking for a whoa in the last picture? If so, then your hands should be lower. Good practice is to try and keep your hands at the bottom of the swell. If your guy has problems with whoa'ing, I would definitely start training with the one-rein stop. My mare could be a real hard head when it came to the whoa on occasion, a couple of one reiners encouraged her to be softer and listen to my vocal and seat cues.
Otherwise, really looking good. That horse is just too cute for his own good