A handy half pad to have is the AirPad from the makers of the Flair system. It has 4 chambers of air and a small hand pump. It sits under your saddle just like a regular half pad but you can adjust the amount of air in each chamber with the pump (you can even do it while you are in the saddle) and even out how the saddle is sitting. It even works to even out how the rider is sitting if they have issues with being crooked. It is great for horses that are growing and haven’t finishing developing or horses that are crooked. It’s not cheap but worth it. As long as your saddle isn’t pinching (too narrow) on your horse, it can help.
Good luck! I had to have my horse and saddle fitted every two or three months between the years of five and eight until he finally grew into himself.