I attended a hoof trimming clinic on the weekend, and I learnt a ton. I'll share with you what we were taught. The person doing the course, Andrew Bowe, is a master farrier and certified race track farrier - he has now converted to barefoot and specialises in rehabbing cases of chronic lameness (Navicular, laminitis, neglect) using barefoot techniques. I found it very interesting that someone who has shod for years, well, has now on his own violition, switched to a completely barefoot approach. Of course, this clinic was taught from a barefoot POV, but all of what he said made sense to me. I don't think I will shoe any horses again - I had only shod one horse on the fronts many years ago and all of mine since have been barefoot, but not 'properly' barefoot - just farrier trims.
Firstly - In regards to the terrain. A healthy, barefoot hoof can travel just about any terrain. Horses survived being hunted almost to extinction many, many years ago by surviving in places so arid and harsh that men could only survive in nomadic tribes, not numbers big enough to have enough hunting power to wipe them out. They travelled many miles a day in pursuit of food and evading predators. The healthy hoof is designed to travel many miles a day, on rough terrain. It's what it is designed to do, and it does it very well when managed properly. So if you can get your horses feet functioning as they should, he should have no issue on the rougher terrain.
How it should function - The hoof should always land heel first. This is because there is a set of 'landing gear' in the back of the hoof - the frog combined with the digital cushion which is a pocket of fibrous cartilage. These two combined act as a shock absorber - They spread outward and convert the energy from an impact into heat energy in the blood and pump blood throughout the hoof capsule every step. This circulation of blood stimulates the growth of cells, which leads to faster growth of the sole and frog, and a thicker sole. So in essence, the further the horse travelled, the more circulation was enabled, and the faster the hoof grew to accomodate for the extra wear. So there was a kind of symmetry - The more movement, the more growth - the less movement, the less growth. For this to happen, the hoof needs to have a short toe and the frog needs to be touching the ground - To do it's job of absorbing the impact. This is where the principles of barefoot trimming come about - The two main principles we were taught were as above - Short toe and frog on the ground. The feet of wild horses who travel many miles also showed us another few points of how the hoof was designed - There are two layers of hoof wall - the inner wall and the outer wall. The outer wall is not designed to bear weight - This is why you see what is known as the 'mustang roll' - The outer wall is worn down and the weight is born on the inner wall. It assists in breakover and prevents chipping. The other point is that the quarters are not designed to be weight bearing - this is the sides of the hoof, between the toe and the heels. Not entirely sure why this is, but it is. Lol.
So, if we trim to these parameters, we can get the hoof functioning as it is supposed to, with the shock absorbed by the landing gear, which facilitates circulation and will speed up growth to accomodate excess wear.
Now, this can be disrupted by many things, namely the fact that our horses don't travel many miles in a day and they are often on soft ground. Many aren't trimmed properly or are shod, meaning the frog isn't on the ground and the landing gear can't do it's job. This generally means the horse will change to a toe first landing - Which is not good.
A toe first landing sends all the impact up through the wall of the hoof and straight into the joints of the pastern/leg. These joints have only a thin layer of cartilage and a thinner layer of joint fluid seperating them - they are not designed to absorb impact, after all, that's what the landing gear is for. This is the main cause of most chronic lameness and minor lameness - These joints tryign to absorb shock they should never see. This is why you often hear that studies show a shod hoof takes more concussion at a walk than a healthy, functional barefoot does at a walk - Because the landing gear isn't being utilised.
Because the frog/sole aren't touching the ground, the structures inside the hoof can start to drop, seeking that contact. The wall is not held fast to the innner structures - The inner structures are almost 'floating' within the hoof capsule, so when the support of the frog and sole is taken away, they begin to drop to try and find it again. This is when you will see flat soles developing.
Another consequence of the landing gear not touching the ground is compromised circulation, which in turn leads to slow growth and thinner soles. Because it is not being used, the frog often compacts. Many people notice how much quicker the hoof grows when shoes are pulled, as the circulation can begin to function again. The shape of a healthy barefoot is vastly different to the average shod hoof - The barefoot should be almost circular, with a large frog that is wide at the bottom, a short toe, and a concave sole - A long term shod hoof often has a flat sole, small, contracted frog, a longer toe... All things that are proof of the foot not functioning as it should.
Now on to boots - Boots are handy when making that transition from shod to barefoot. You obviously can't just hack into the heels and sole to get the frog touching the ground - this would cause immmense pain. Boots are used along with specially designed pads for any issue. If, as is usually the case, the frog is not on the ground, then you would use pads with a raised frog section so that the frog can get back to it's job of absorbing shock and enabling circulation, which will speed up growth of the frog and sole, and it will soon grow out enough that the pads can be taken off and the frog can function nirmally again. Pads can also provide support to the sole, so it doesn't drop or flatten further. The boots themselves protect thin soles while they are thickening from getting ouchy on rougher terrain.
In regards to you changing paddocks - That is great. The feet will be passively conditioned to the terrain - it is often said that if you want to ride on it, they need to live on it. Soft ground feet are vastly different from hard ground feet, and if the feet are conditioned already to the riding you want to do, it should be no issue.
Phew, that was a novel! Obviously you can tell that my opinion would be to give barefoot a go - Your success will depend on how long he has been in shoes.
That is just my opinion and what I learnt from a one day course (My mind boggles at how much I still have to learn) presented by someone who believes fully in barefoot. Go with your gut and good luck in whatever you choose!