Hi OP. I just read through this entire thread for the first time, with interest - it’s certainly been an interesting “roller coaster ride”, and thanks for providing such specific details!
Some thoughts: re: his feet, you might try using Keratex on the soles as a hardener - it sounds like the outdoor ring footing is harder and more abrasive, so it may be wearing his feet down quicker (and he also may be getting more ouchy as a result), though a hoof supplement like Farrier’s Formula - or something with Biotin, Methionine and Lysine - would be a good idea in general. I have an older mare now retired (22) who I did a lot with, all barefoot; eventing, dressage, hunters, jumpers - but she has exceptional feet with hard, thick soles. If he has good feet and has been barefoot all this time with no issues, a supplement and a hoof hardener might turn the trick. (My mare was only shod for 2 years starting at age 11 because I was running Prelim and needed to use studs for traction. Her hoof quality was already excellent, but I also fed her flax because it helps them grow a good foot. You might try adding flax to his diet, it has many benefits
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(Two of her hooves are white - the old adage that “white feet are softer” isn’t actually true, they just show bruising more easily due to the lack of pigment which is probably why the old wive’s tale persists…)
Keep in mind that it will take awhile to see results with a supplement (it takes them 9 months to a year to grow an entire hoof) - but first try the Keratex (or Venice Turpentine) and see whether that helps. Horses should be shod IF they need the protection for the footing they’ll be working on (and there are other reasons), but if you can keep him comfortable barefoot, all the better.
Diet: there are horses that react to MSM, take him off of it immediately. I have used it on my last 3 horses with no ill effects (and I like it for its general anti-inflammatory properties), but it can definitely cause crazy behavior in some horses. Otherwise, a good RB along with flax (and alfalfa cubes) should cover his nutritional bases if he’s in good weight and on good pasture, supplemented with hay? If he needs additional calories now, you might try a fat supplement, “cool calories” rather than a starchy grain with a high NCS.
I think you are onto something with the herd status thing! (Good for you for thinking outside the box.) One of my horses was one of these, what I like to call an “insecure Alpha”. Those that are BORN Alpha (like my two mares, mother and daughter, sigh) are generally more secure - but an older horse who is “suddenly thrust into the role” can become VERY insecure, herd bound, and spooky; they become obsessed with their herd because that’s their new “job”, but they aren’t completely comfortable with this new gig. Ground work is your friend in this case, get his focus on YOU, move his feet, get his brain working, work on getting his head down so he can drain off that adrenaline response, breathe, relax. Once they are in that head in the air, snorting, adrenalized, reactive brain state they are VERY hard to reach!
See Warwick Schiller for this
I have done a lot of TTEAM with my horses when they were young to help them to turn on their thinking brain, drain anxiety, relax, focus on me (and on their feet), they have many simple exercises that may be beneficial.
I use longeing on occasion, but it’s not my automatic go-to. If you just run them around to get the ya-yas out, this can result in a fit horse with a tight topline who spins around unbalanced when on the line (not helpful for training), though it sounds like your guy is pretty chill on the longeline. I instead use groundwork before getting on for my older, more trained horses if and when I feel like I need to get their “head in the game” and their focus on me (or if I feel like they may start out distracted); you don’t have to spend a lot of time on it, just 10 minutes of it, watching your horse for signs that he is relaxing, brain and body - licking, soft eye, dropped head, soft topline - and now receptive to going to work (you know him well so that will be easy for you to determine.) I do gentle lateral work from the ground both to soften and supple and also to get the horse more responsive to the whip aid; mentioned earlier in the thread.
I am a little concerned about the breathing issues from your descriptions. Might be work getting the vet to check him out, and there might be a medication that will help with this (this is not normal!)
Another thing that can produce sudden behavior changes is ulcers - either gastic ulcers or hindgut ulcers (acidosis), which can be triggered by change in feed or the sugar content of the grass. Something else to discuss with your vet. Change of feed, change of work, change of environment (new barn, new herd), any and all stressors - can trigger ulcers.
Chiro and acupuncture might be helpful for his soreness (and have the saddle fit checked), but if he his glutes are sore, that is is good(ish) sign; though it also may be muscle soreness from the overly deep footing. Good to hear that they are changing it out, that can be very dangerous to work in, muscle soreness is the least of your worries (tendon and ligament strain, already discussed on here.)
As for the riding, it sounds like you have been making steady progress up to this point but there is clearly something “amiss” ATM, possibly relating to one of the above ^^^ things.
You are a responsible and caring owner to be so committed to your boy, you clearly love him and have a good relationship, good for you.
Best of luck!!!