Ello... i'm a new user... Skye... was just about to post a question myself when I saw this one. One of our horses used to have extremely weak feet... typical thoroughbred... and it seemed like we were constantly ringing the farrier to get him to come out and reshoe him. It got recomended that we try bluestone and water... and it worked wonders! After a few weeks his shoes were holding for at least 6 weeks. I'm not sure wether it will help in your situation, i.e. wanting to take the shoes off and harden his feet, but it seemed to help our horse hold shoes, so i guess his hooves overall would've been stronger/healthier.
If you want to try it, just buy some bluestone, put a couple of tablespoons in a spray bottle and mix it with water. If he's got shoes on, you know the mix is stong enough if the shoes turn a pink/coppery colour. Done! For the first week, apply it every second day, and after that about twice a week. Good luck!