Britching and breast collar.
Most folks have trouble like this when trying to fit a mule to a saddle even if the saddle has mule bars.It is something that mule riders are faced with all the time. ALthough it is usually no withers and a flat back from front to rear.
Also has he been over fed. Kind'a sounds like it, but can't say as there are no photos. I like to be able to feel a few ribs,but not see them pronounced.
How much space is there between his withers and the gullet? Should be about two fingers width. Is the center of the skirting on the back of the saddle riding on his spine?
Put the saddle on him with no pad, but put a piece of twine down the center of his back before doing so. Get on and have someone try to pull the twine out the back.If they can't that means the saddle is setting on his spine and not the rib cage and the bars are too flat.
If it is perched up there way high,the bars will be digging in at the bottom and the saddle is has too much angle on the bars.
It's not uncommon to have to shop for a saddle with a new horse
