Ask why you think he's sensitive? Does he have a thin sole? (can you put your thumbs on each side of the frog apex, press and see give?) Is thrush making him feel like he's walking on diaper rash? (frog wide, smooth, with no sulcis crack, but merely a thumb-like depression on top of the frog?) Wet, soft feet go with this one...deep crack, ratty/ atrophied frog. Are any bars excessive and jamming into his foot like stones in his feet already?
All these things point to the need for a well developed back of the hoof, good balanced trim and lots of movement with a heel first landing. It is this strong development and thrush-free environment and sole allowed to grow thicker that takes you away well away from this. Are you feeding trace minerals?
In the interest of growing a thicker sole, I would not burn it or pickle it with formaldehyde. I think that's a horrendous thing to do to a live entity. Neither would I allow the farrier to carve into it. I would use boots with pads to develop and be able to go across gravel without a wince and be abscess free in the process and wait for the sole to grow thicker and going after thrush.
Gravel is graduation in tolerated ground surfaces...the ultimate goal, not the beginning. You can't take a horse from a cushy environment and expect him to be sound on gravel. It has to be earned. Pea gravel laid down in places where the horse loafs, has to pass through or around the water tub will help will start to give you that development that's needed on other ground when in work. The environment on top of gravel is also nearly thrush free as well and helps with that battle and develops and toughens a hoof and improves it in between rides.
These are things that promote good hoof health...that has him laughing in the face of anything that comes along.
If you see a horse wince on any surface, then know that you are in a higher risk of entering abscess world and about to be sidelined. The boots and the gravel will develop and protect and keep you going in the meantime.
I see conditioners and emollients as instant suffocation to the hoof's ability to take up and lose moisture at will and paints as instant death. Promoting good hoof health is the answer and what the horse really needs.
Hope this helps....