He's behind the vertical in many of the photos, and your hands look tense, so I'll assume you're pulling him into contact. Remember correct contact has nothing to do with you taking the slack out of the reins and feeling his mouth. Correct contact is when there is slack in the reins, and the horse moves to take the slack away. If he doesn't, then the contact is not correct!
This is why in some shots his back looks tense--his throatlatch and TMJ are tense (behind the vertical) which is translating into the tense back, which will leave his hind out behind him. He's a beautiful boy, so start him on some correct long and low, and leave the contact out until he's ready for it! Him being hot or nervous is no reason to scoop up the reins... work on him being calm first (which long and low will help him with).
Good luck! If you need anything in more detail, just let me know ;)