He needs a hoof supplement with at least 20mg of biotin. I use BioFlax from Smartpack, or SmartHoof. If you have more money, you can check out other supplements that have extra ingredients.
I'd invest in a pair of hoof boots. I'd suggest Easyboot Gloves. They're easy to put on and can stay on for a decent length of time. They are also rather inexpensive. This will help with any bruising the horse might have. You really need to be careful about bruises when transitioning from shoes to barefoot, as bruises can because abscesses, making the hors super lame and in a great deal of pain.
Once his hooves have more moisture and look decent, you can spray iodine on his soles daily to help toughen them up. Iodine kills living tissue so try not to spray on the frog or heels. After a trim or when you think he has a bruise, you can paint Venice Turpentine on the soles too. This quickly hardens hooves. Do not apply more than a few times a week though, or else the hooves become too hard. I try to not apply it within a few days of a trim either, because the farrier will hate you if you do. He has to work harder.