Well in fairness to those who’ve posted, you do have to admit that the picture where his head is raised does make it look as though his back has some pretty significant curves.
Maybe the real challenge that the back poses is a substantial change in its shape from its resting profile to the working profile.
If that is the real challenge, then might be very difficult to do a simple remote fitting, because the fitter would really have to see the horse in motion with the saddle on its back to tell if it actually works.
Good luck!