Actually, if it's just lack of muscle, then a pad can definitely help. Skito correction saddle pads
are great for filling in for muscle. The shims can be changed out as the horse develops, or removed entirely and used just as a shock absorbing pad once the saddle fits well again. You use the correction pad over a regular blanket or thin pad.
If you don't want to use two pads, then you can order shims for the regular Skito Equalizer saddle pad
. It's a full pad, made like a traditional western pad, with pockets for foam panels and shims.
If you send in photos, they can tell you which shims will work best, or if the problem can be addressed with shims at all. Take photos of the horse with no saddle and with the saddle, but no pad.
My DH's gelding is half TB and has issues with western saddles being too low on him. A set of shims in his Skito pad levels it out and keeps him very comfortable.