Okay, well, coronitis is an autoimmune disease that is chronic, meaning it will never go away, and will in all likelyhood cause severe lameness eventually. It's treatable, not curable, worldwide. In New Zealand it's thought to be caused by spider bites (but that isn't proven).They really don't know why it happens. Most of the time it's thought to be allergies, etc before it's diagnosed. It can affect all the skin on the horse, but in coronitis, it's specifically in the coronet as hoof horn is specialized skin cells, and is also likely visible in his ergots (funny nubby thing in the fetlock) and chestnuts (bald skin callouses above the knees and below the hocks).
However, ANY inflammation that would occur with that is also causing laminitis, maybe not Acute, Severe laminitis, but laminitis it is. The technical medical defininiton of laminitis is inflammation of the lamina (connective layers that attatch the hoof wall to bone), it's not always the crippling, walking on eggs, and penetration of the sole, kind. Any growth rings on the hoof wall are scars, you could say, that are where the lamina attatchement failed, this is why there is a ridge. At some point, the wall is trying to grow re-attatched, and will grow exactly in line with the coffin bone. Horses that have "grass rings" have laminitis rings! Any rippling of the hoof wall is sign of laminitis, yet the horse may never take a bad step.
And since I'm giving a text book response, your horse is foundered. Meaning,in general, there is rotation. No horse is born rotated! It's a pathology developed from trauma, crappy farrier work or diet, or disease, or any combination thereof.
I never said 2 simple trims would cure it, but one good trim can set the coffin bone back in the proper postition, relieve the leverage that rips the compromised wall away from it, and allows it to grow down reattatched, with regular trims following up. The hoof isn't healed until the whole thing is regrown reattatched, and may take as little as 8 months or as much as over a year, depending on the situation,if the other factors are corrected.
Shoes are usually much less successful in correcting the problem, as they suspend the hoof capsule by the lamina, that is weak already, and promote sinking of the hoof bones into the hoof capsule. Laminits, as you said, can be caused by coronitis, so as long as he's suffering from that, even with treatment, the prognosis is not great for him to be sound, EVER.
The hoof growing straight down is where the healthy, attatched wall is, and tells you how the coffin bone is oriented within the hoof. Trim to make that angle match (and that would be the normal angle for him) and relieve the toe and you will be on the road to a happier horse, until his next flare up, at least. Sorry to say, it's just likely to be a life long battle for you and your horse, and you may never know why he has the flare ups. I wish you luck and hope research finds answers soon to help him. I can point you an intresting article about it by Derek Knottenbelt that has done a lot research in England about this.