Cool video and you guys are a really "nice picture". I see you as being a little "busy" with your hands. I think you're trying to get him to give to the bit, come to you, but you're not finishing the job. I think you need to think more about moving the bit in his mouth. I'd try pushing him forward into the bit while bending his head to the rail and then back to the inside. Slow, steady, determined movements. Like horseaddict said, make sure if you're flexing to the outside that you give with the inside rein. I'd also try some zig-zag patterns, moving on and off the rail, at a diagonal across the arena. He could be bored with circles.

Let us know if this helps!

Are you trying to collect him here, is that it? I'm working on this, too. My only suggestion would be to not jerk that inside rein...you do a good job of pulling from your shoulder, but there seems to be too much slack/not enough contact on that rein before you try to work the bit. Keep the outside rein as a "wall" so he can't bend his neck (pull your outside rein to your hip), then open your inside rein a bit and pull gently from a solid contact on his mouth. When I do it, it always seems to me that I am pulling against myself. And ask for the inside bend with rhythmic nudges from your inside leg. You'll know you're doing it right when he starts softening of course, and then foams at the mouth and starts "chewing" the bit as he tries to swallow. When he starts to soften, reward him by letting him walk for a couple of steps without working the bit in his mouth but ask him again right away after that, especially if he brings his head up again.