Are you happy with your farrier? What do the hind feet look like? IME, the feet can often be the root of these kinds of issues. I had used the same farrier for almost 30 years on all of my horses when I finally changed to a very well-respected therapeutic barefoot trimmer/farrier. She discovered that my guy had NPA in his hinds and began the process of correcting it. That along with getting a saddle that the horse approves of (even though the saddle at the time had been fitted and deemed to “fit”…horse didn’t think so). I also added his beloved Cashel foam wedge cushion pad (that I had stopped using because I bought a Mattes pad that I thought would be wonderful…turns out he prefers his $45 half pad to his $300 saddle pad, of course).
So…feet fixed, happy saddle and pad combo, and then nutrition: plenty of magnesium and MSM for him.
What else? Lots of butt tuck and tummy lift stretches. Lots of backing (both on the ground ad under saddle), and working over poles when possible.
And I just bought the equicore/equiband system, and that really seems to help him use himself correctly to build the right muscles while working. I’ve only used it a few times so far, but he actually seems to like it.
But honestly, if I had to pick three things that truly are non-negotiable for helping my guy: fixing his feet, figuring out the saddle and pad combo that makes him happy, and magnesium. I feel that if any of those three weren’t in place, I’d still be battling the issue.