Rising in the back suggests your horses shape has changed. He has probably lost condition and the saddle has dropped in front causeing it to rise behind. I would suggest getting an approved saddle fitter out to check it. I hope this helps ☺️
It does help to use the back two billets on the saddle. It helps stableize the back portion of the saddle.
As horses age or their workload increases/decreases, their shape changes.
Maybe a loss of weight or. The girth is too loose or maybe just that it doesn’t fit anymore. He’s outgrown it
How long ago was it fitted? The horse has probably changed shaped since fitting :)
There are nine saddle fitting points. One of them is full panel contact. When your horse is groomed, put the saddle on without any pads. Take the palm of your hand, and run it down the length of the back under the panels of the saddle. If you feel any uneven pressure, your saddle does not fit. If it’s lifting off the back it probably means it’s pinching at the withers and is too tight for your horse and you don’t have full panel contact. As you’re horse’s body changes with his muscle, the saddle fit may change as you’re horse changes. Have a saddle fitter come out again. In the mean time, use a half pad (Mattes, Fleecework, Oglivy) or use a riser pad. Hope this helps!!