Well, he's definitely hollow through the back and your position is thrown back.
Look at this photo. First, the horse's body language is saying "OWE!" Head high, neck tense, shoulder's tense, back dropped, and hips "fidgety." The saddle looks too low in back, you're sitting too far back, your legs are out in front of you and braced, your hips and shoulders are tense, and your knees look to be pinching. You can see this same reaction in you and your horse in most of those photos.
Oh boy, he's really PO'ed at the canter here.
I would give him some time off, let his back 100% heal up. Do some stretching with his neck and legs. Give him some turn out time everyday to run and roll around.
So we can check his saddle fit, put the saddle on him without a pad. Girth it up just lightly (so it stays in place). Take side, front, and back shots. From the front, stand away from him, just to the side, to get an angled shot showing how the front of the saddle lays against his WHOLE shoulder.
Without seeing the above photos, I'd say the saddle is too tight in the front, causing it to be too low in the back. This throws your position back, which causes extra pressure on the horse's lower back. The half pad and poofy square pad is only making the problem worse.
I'd also suggest some lessons to help your position (though not until you get a saddle that won't fight you on it). You need to open your hip/pelvis, bring your leg back under you, lift UP your hands (bend your elbows), relax your back and shoulders, and try to keep your center over the horse. Your center is right in the middle of your lower pelvis. You'll feel equal weight on your two seat bones and pelvic bone. You only need to tuck your rear and ride your seat bones when you need to drive your horse forward or sit a very energetic working trot. Otherwise your seat should be in a "neutral" position. Once you can open up your hips and get your leg back (don't push your heels down, you're riding Dressage, not hunters!
) I'd like to see your stirrups 1-2 holes longer. There's nothing more elegant than a tall rider with a LONG and relaxed leg