Looks pretty graphic, but isn't that big of a deal. Yes, it's growing in nicely!
Most likely he'd has an abscess in there, and it blew out, leaving separated material (wall) just barely attatched. It sloughed off, painlessly, like an old scab. It actually looks like a surgical "resection" that is done on extreme laminitis cases! What you see under there is his white line. But don't panic! As long as HE'S okay with it, you should be too. Obviously that exposed part would be more senistive if he struck a rock, but basically it's fine for now.
It's kind of like if you've ever smashed your finger hard enough for the nail to turn black and fall off. It grows back, but after the initial pain, it feels fine, but looks nasty.
It will take a few months to grow the hoof wall down completely, and a good hoof supplement (like you are giving) will help, as will plenty of exercise. I trim a couple of horses that have old wounds in the coronary band that scarred and they don't even have entire hoof walls (in the quarters). It' s just exposed white line that will never grow horn again. But they are barefoot and compete!
Keep up the good care! His foot is fine.