What findeight said. I can’t imagine any topical application making a hoof “grow” significantly faster than they already do. Hooves are made of keratin, which is a protein, so the only logical things to do are to make sure your horses diet has the correct amount of high quality protein, and then try to protect his hooves from environmental conditions (ie mud and water) that cause the hooves to become dry and flaky.
Chronic poor quality hooves can be an indicator that your horse’s diet is insufficient in protein, meaning the quality or quantity is not enough. OP, are you completely satisfied with his coat condition and body weight? Is the horse bursting with health, and just has bad feet?
I get ottb’s in for r&r and they invariably have bad feet. My blacksmith comes out and moans and groans and talks about possibly having to put glue on shoes and patch places, but after 60 days of a premium feed (TC Senior and TC 30%), and my diligence in fighting the drying effects of mud and water, we see dramatic improvement. What we see is that the horse had OK feet, but the constant exposure to water (they love to wash legs at the track) caused chronic brittleness and cracking.