That he is releasing his jaw; this is good! Much better than a horse that braces the jaw rigidly closed and leans on the reins.
Ask for the backup with one rein a bit "active" , like kind of ticklying him a little or gently squeezing that rein like yuou'd squeeze out water from a small sponge. The other rein is kind of steady. If you horse puts a little bend in his poll to the side, this will help unlock his feet. Be patient, look at his ears and eyes and don't release the pressure until you see a change. I want to see my horse cock an ear back and maybe roll an eye back , if he lifts his head a little, all these things indicate that he is thinking, OH, now I place my feet in the way that backs me up. If you feel his shoulders lift a little, he will probably step more lightly backward. Don't wear him out doing this endlessly. Too much backing up on a horse kills forward.
I use backing up to get my horse's mind back with me and kind of test him to see if he's listening. I use it to get him to stop leaning on the bit and going througth my hands and I back up, especially back up a hill, to build belly muscles which helps build topline .
Eventually, this will be good for Skye and you help him rebuild his back/neck.
Little by little.