I agree with drummer. It's obvious you are working down the right path, the horse is experimenting with sometthin new to him; stretching downward. He seems to be doing it a little bit like he's "barging" on the rein, but at least he's trying and that deserves acknowledgement and reward, as you do.
I would shorten your stirrups just one notch. It looks like you are fishing for them at times. This might help you be better alble to keep the lower leg still while posting. And, it might help you be eventaully able to sit upright, even when he barges down, so that you can then use your abs to lift him forward, and encourage him to drive into the bit, even while reaching downward and forward with his head. Right now, you gt him to put his head down, but you dont' worry too much about the quality of his contact to the bit while he does this, but in time, you will want him to "carry" the bit down, stay there, push up onto it ,from his hind, and then follow it back upward as you lift him back up.
It will help , as drummer said, to stay on a 20m circle instead of straight, bend, straight, bend .
He's a cute horse and you are doing a good job!