Well, this is just from my knowledge base but I think the crack isn't healing for a couple of reasons.. first, the hoof will continue to crack until you relive the pressure on the crack itself. This has more to do with how your farrier trims your horse than the supplements you give. The hoof looks quite healthy, but you can't tell much from photos... so I would say give your farrier a call and get him/her to either shoe your horse or put some sort of pressure relief at the top so the crack doesn't keep moving up as the hoof grows out.
I hope that made sense. I'm not very good at explaining things in text.