Is your horse pastured in a wet area? Mud, wet, or wet stall? Is there any black color gunk when you pick his hooves? Is there an odor to the hoof, especially when you pick the hoof? It may be that your horse has a fungal/bacterial infection such as Thrush. This can come about in moist environments, living in a paddock where manure it not removed daily, or not picking the hooves daily. The horse's foot conformation could also be set up to hold moisture therefore contributing to the problem Please let me know the answers to my questions and I can better help you. Have you had a vet look at it yet?
I would say that his pasture would be a perfect breeding ground for him to have Thrush, but at this point, if he does not have it, I would say that the moisture is softening his hooves which is resulting in the white stuff you see when you pick. Imagine your hands in water after a long bath...how soft and funky your skin and nails become, and at some point, sloughing would begin. If you could get his pasture drier, or move him to a drier location, it would be best for his hooves in the long run.
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