that's a good video! short and showing the pertinent points without obnoxious music or fluff! well done.
you have a very nice, sympathetic seat. by that I mean you ride soft and solid, (no bouncing around), not hard in the saddle, no driving. you sit very tall and stable but not hard, your hands are steady as a result of your steady seat. you tend to ride a bit light and forward in your seat, as if you were riding a yougster. (is he young?) all of this makes for a horse that finds it easy to go forward.
the problem is, that with this horse is finding it so easy that he isn't putting in enough energy, and your are not utilizing his energy as well as you could. I mean, your hands, while very good at following his motion and rock steady in position, are a bit too far forward, and too "allowing". with your elbows out, you are allowing too much of the horses energy to fall forward. it's , again, almost as if you are riding a young horse who is unsure about going forward and you are being a bit "dainty" about taking a contact on his mouth for fear of destroying that forward.
But, since he is a heavy horse, he MUST work harder to be able to do energetic and balanced transitions. he must push more, and you must contain more , in order to translate some of the "forward" energy into "carrying" energy.
#1 bring your elbows in and connect them to your core. ride more from your core.
#2 allow with your hands only when he has given you more energy, as you've asked. and only after he comes up a bit softer on the bit, with a bit of lift in front.
the good thing about heavy horses is that when they do soften on the bit, and raise their front, the change in feel is really dramatic.
all in all, this is a picture of a nice pair who work together well with tons of potential.