From what one can see in the photos, the feet look to be in good condition, well maintained, with no visually obvious reason that would suggest a problem.
As already observed, the horse does present a camped-under stance, suggesting the possibility of caudal heel discomfort.
You provided additional insight, to wit...
The horse is very tender this year on stones and gravel, hasn't been a issue prior to this.
Yes, he doesn't travel to areas that are really rocky. I ride on local trails behind my house, hard packed sandy loam, hard surface but not very rocky. Loose sand. It's a easy walking surface. There are a few spots on trail heads that are course gravel, he has always walked softly there, this year is very softly and very obvious when he steps on something painful.
I hear this a lot from owners of barefoot horses that are used for more than just a pasture pet.
What the owners want
to say is, "He's generally sound while barefoot".
What they really mean
is, "He's sound... except when he's not".
It's a subtle but important distinction.
A horse at liberty (e.g. Pasture pet, wild horse, feral horse, etc) has practical advantages over a horse in use.
- The horse gets to decide when it moves, where it moves and how fast it moves.
- The horse doesn't have to carry an extra 20% of it's weight in the form of rider and tack.
Put a rider on that same horse and the animal loses those advantages. It now has to submit to your will in choosing direction, terrain, speed. It has to manage an increase in load that may or may not be in good balance all the time.
These appear to be both simple and obvious observations, but it's remarkable how often horse owners either forget or simply dismiss these factors in determining suitability for a particular use.
The presumption is, if the animal has nice feet and is generally comfortable/sound at liberty or even under saddle over forgiving terrain, then the horse is considered to be a good barefoot candidate for a broader spectrum of use.
The trouble with that presumption is, it isn't always correct. I think some owners struggle with that reality. The situation is worsened if someone else shares their story describing how they've always kept their horse barefoot and the animal is a rock crunching monster truck that can carry any load over any terrain. Surely, if little Sally's horse can do it, then all horses can do it, right? All they need is the "right trim".
Of course, the answer is, no... not all horses can. Many horses need a little help so they can better manage those 'disadvantages' described above. Some horses need a lot of help and some can't do it no matter how much help we give them.
Your own situation presents two distinct possibilities.
First, the horse may truly be sound at liberty and generally, but not always
, capable of remaining sound under saddle. His comfort level under saddle may be directly related to the challenges of terrain or performance expectations. Increase those challenges and your generally 'sound' horse is subject to problems. Give them a bit of protection and the animals ability to comfortably overcome those challenges improves.
This first possibility is the easiest problem to explore. Put a set of shoes on the front feet and in the vast majority of such cases, all the problems are resolved.
The second possibility is a progressive pathology.
If the horse has generally done well for you over the years, but you have noticed a distinct change in performance (soundness level while in use) over the last year, then it becomes appropriate to explore pathology related lameness issues.
The 'camped-under' stance warrants a thorough lameness workup, including a set of radiographs. I'd want a 60 degree oblique, d/p and lateral xrays, focusing my attention on the distal interphalangeal joint with emphasis on the navicular region. Look for a positive reaction to a flexural exam (navicular disease diagnostic) and follow with a thorough hoof tester exam; emphasis on the caudal aspect of the foot. The lateral radiograph will define phalangeal alignment, sole thickness, any arthritic changes and spatial relationship of the distal phalanx in the hoof capsule. The oblique and d/p provide information about the navicular region, the dipj and condition of the coffin bone.
If I were working on this horse, without benefit of veterinary diagnosis, I'd start with a basic pair of front shoes. If the problem persisted, I would recommend a veterinary work-up as described above.