What is his diet? Diet plays a HUGE role in hoof health.
Genetics also play a huge role in hoof health. I have a mare who simply cannot be worked barefoot even though I have done everything in my power to change this.
These are the steps I take:
1. Make sure the horse is getting a correct, balanced trim and you keep up on them with regular trimmings. (I can go 8-10 wks between trimmings in the winter, but I have to have them done ever 6 weeks in summer.)
2. Get the horse on a good, low NSC diet. This is extremely important as high NSC feeds have proven to create all kids of issues including poor hoof health, ulcers, IR, founder, laminitis, etc.
--My feed recommendations are: If horse is sn easy - med keeper, a RB such as Buckeye Gro N win (I use) or Triple Crown 30%. If you find you need some added calories to keep him at a good weight, then add extra calories in the form of alfalfa pellets, rice bran pellets, beet pulp, etc). If he is a med-hard keeper, then it may be more cost efficient to go with a different feed such as Triple Crown Lite, Buckeye Safe N Easy or Triple Crown Sr (listed in order from least calories to most calories).
3. Put the horse on Simmerdown. This is a product that I have just recently been made aware of by a friend of mine. I have seen the improved hoof growth and health since the horse has been on it. I am in the process of ordering some to try on my mare with thin soles and walls who cannot go barefoot when in work. You can order it from www.horseherbs.com
If none of the above works, then I would entertain the idea that genetics simply wont allow this horse to go barefoot and I would request the help of a GOOD, COMPETANT farrier.