Soreness is normal when transitioning. His sole will harden up. I wouldn’t give him the standard barefoot trim for a week or so. Let his sole build up some calluses before you cut the wall down, if you leave the wall a little long it’ll help put a little space between his sole and rocks while it hardens, that way you don’t have him walking around like he just foundered. Yo can’t leave the wall long for to long though because without the support of the shoe most hooves will start to crack. I use iodine on the sole. Depending on his hoof quality you may need boots for riding on rocky terrain.
You also have to realize that not all hooves are made equal and his breed has nothing to do with it. If he has soft hooves he may need shoes. Get a good barefoot trim. You should see a concave sole, with the walls level with the sole. The heal is cut down level with the frog and the front of the hoof is rolled forward. Walls are rounded up(this helps prevent cracking). Watch him for a few months. His feet shouldn’t crack as long as you are not leaving the walls to long, but if they still do then you may have one of those horses that needs shoes.
Every horse is different and so are their hooves. One way of doing things does not apply to all so don’t buy into having to be barefoot, shod, or wear boots. Do what works best for your horse.