One thought to add here as I see your posts about your saddle - maybe try the EQ saddle rehab program? You can rent a saddle to see if it helps with back issues. If you like it, they give you a credit to buying it. I have an EQ for both of my horses and find they are super back friendly. They are also kinda “self-adjusting” for minor strength changes and are very stable. I find this true for both my super wide mare and my narrow gelding. (I have different tree shapes for each.)
I used to have a KS horse until we had to euthanize him for an unrelated reason. He was a total dream as long as he did walk/trot only. So there may be a future for your boy, even if different than what you had planned.
Also, I think you were on the Zycosan thread, too, right? I’m planning to keep my gelding on Zycosan quarterly - I start round 2 this weekend. If it helped your gelding, maybe think about doing it more frequently? I’m hoping there may be some kind of build up in their systems. No research finding that that I’m aware of, but one can hope.