I recently bought a horse with similar feet issues. I was told biotin, copper and zinc.
First thing I did was change his diet(he was being fed low quality hay and sometimes straw). He went to a high quality grass mix and triple crown senior grain(high copper and zinc), along with a biotin supplement and horseshine(omega 3). And I got my farrier out asap.
I use Durvet’s Daily Biotin(red and white container) for all my horses. It’s $14 for a months supply(for 3 horses), so you’d probably get 2 months out of it for 1 horse. http://www.duboisdistributors.com/catalog.asp?prodid=669212&showprevnext=1 This was the highest biotin % I could find that wasn’t out of my price range, since at the time I was feeding it to 2 horses and now 3.
He’s had 2 trims and his feet look 100% different. They don’t get all chipped up anymore and his soles look a lot better as well.
Even just changing his diet is going to do him good.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."