I noticed the crack in the picture a couple weeks ago ago. The size has increased some, but seems to be stabilized and is not right against the coronet band. Wondering if it is just and abscess working out our something more significant.
I usually do my own horses, so I don't have a regular farrier, but the first guy that looked at him thought it was an abscess or broken bar. He recommended modifying a shoe with a cross bar then cutting off the heal to take weight off that part of the hoof
If you can get some, and I do this on occasion with my draft hooves as they have done this, if you can get your hands on some bee's wax, shave off enough or a good chunk, get it nice and soft and pliable, then work it with your fingers by pushing it as deep into the crack as possible. The honey has an antibacterial property to it as well as healing properties and it will protect the hole from dirt and rocks.
If you don't have any or need some, shoot me a pm as I have a box full and can send you some, that is, if you want it.