I can see he is trying to use his hindquarters and back so he is starting to understand that, but it is excessively difficult for him to do when he's so 'tied up' in front (curling behind the bit with a big lack of forward) and will be much more difficult if not impossible for him to do it correctly with him so on the forehand and behind the bit.
Does he always hold his tail out so stiffly? Really shows all the tension in his back as he's trying to hold the 'frame'. Of course, I never seen any other videos of him so I'd have to see those before really knowing if there is a consistent problem with his back.
Just so you know, I'm really not trying to sound harsh. On the plus side he seems very summited to the bridle with a lovely and obedient nature which is kudos to you for cultivating that attitude in him. But in order for you to get him back on the right track you will need to push him FORWARD and teach him to go into the bit instead of duck away from it - and at this point I don't think you can do it in side reins...but if you must...at least loosen them a few holes so that he does have room to properly stretch.
Very handsome boy and I would love to see him when he really starts rounding and using his hindquarters...after learning how to go forward and into the bit first.