Diet & *the way* horses are fed is a hugely important part of health, feet included. As a racehorse, this horse was most probably kept stabled & fed high octane, concentrates. He also may not have been given adequate forage(because you can't have racers with hay bellies!) and was probably fed in infrequent large meals(usually up to 3-4 times daily, because that's what's convenient). Trouble is, horses are built to eat tiny amounts of low grade forage almost constantly. They don't do well with rich feed, or large or infrequent meals, or periods of hunger in between. Oh, also lack of movement(standing around in a stable for hours on end) also effects digestion. Putting/keeping a horse on a healthy, more natural regime is generally all that's required for them to do a whole lot better, including in the hoof department. If the horse has ulcers from unhealthy feeding practices, it can be necessary to treat these too & many owners opt to treat without scoping first.
I think your problem also comes down to the 'to shoe or not' debate, so I'd look into that and learn all you can to make an objective choice(see other thread in my signature for starters), but IMO shoes are also contributing to the problem - and as a racer, this baby has also possibly been shod & run hard since well before he was 2yo. I'd get the shoes off, keep the feet well balanced(usually needs more than 6 weekly trimming) and use boots or something that don't inhibit growth & function, which will protect & support your horse's feet while they grow strong.
I dont see how shoes keep the hoof from growing. It grows from the hairline down.
I think the idea was to put shoes on the horse to prevent weak walls from breaking Tuff. But since you brought it up.... Even if the hoof only grew from the coronet, how do you get that shoes couldn't restrict growth from that point? Shoes aren't just attached to the bottom of the foot & have no effect on the rest. Yes, much of the wall grows from the coronet it seems, but it also grows out from the laminae. One of the ways shoes(or otherwise peripherally loading walls) can restrict circulation & therefore growth is at the coronet - The hoof walls under ground pressure become pushed up and can constrict the coronary artery.