That saddle appears as if it would fit a horse that is a little narrower than your horse, with more of a curve to the topline. It appears from the photos that your horse's back is both too wide and also too flat for the saddle. The problem with adjustable trees is that they only change the front of the saddle. So if you make it wider, it will sit down more where it is supposed to since the channel is too narrow for him. But only in the front of the saddle, so this tips up the back. If you make it narrower, then the panels cannot sit down on the horse at all. So either you will have the saddle tipping to the front and concentrating the pressure in one small area, or else you will have the saddle tipping side to side because it sits up too high on the horse.