Needs to move more forward and to stretch that neck down so his back can come up. I have a horse who likes to be high-headed, so what I taught him to do was to move long and low on the longe line. Did this without side reins; he had complete freedom of his neck. Starting out, he wanted to go around high-headed but I'd jiggle the longe line a little, urge him on forward, and as soon as that head came down, even an inch, I praised him. It didn't take him long to figure things out and now he he does a beautiful stretchy trot on the longe line.
This has translated well into our riding sessions. But the key is to move forward and encourage downward. Ask your instructor about this.
I liked the way he remained so quiet and steady throughout all the banging. And your seat is quiet, too. If you're going to ride with contact, though, try keeping your elbows closer to your sides and your hands closer together, just over the withers, with a straight line from the bit to your elbow. Keep the elbows soft, giving forward and backward at walk and canter, and like a hinge (opening and closing) at the trot.
Good luck with him!