Also I'm having a hard time figuring where the saddle should sit on this horse. I bought a hih wither pad with pads where his withers are sunken in but I still can't figure exactly how far up the saddle should be placed. Those white saddle spots were there when I got him.
From what limited things I can see in the pictures posted, i'd lean towards appendix as well. Please make sure you have a saddle that fits him properly as you can see he's had some saddle fit issues previously.