I am thinking that it is the saddle itself and going to a thinner pad won't do the trick. I'll maybe try to get some pictures of the saddle sitting on his bare back tonight or tomorrow night. But it may be worth a try first.
It could be where it wants to sit on the horse. I have a very large foundation QH mare. Simply assumed she would need wide as she is so wide. Nope. Saddle fitter positioned my semi QH saddle and it works great on her. You may have to find a pad that helps keep the saddle in the position your guy needs it to fit correctly.
Changing name brands may not resolve the fit. Many saddle companies use Ralide trees. If you check the Ralide site, there are 3 basic trees, semi, qh and wide or full. The fit of a Ralide is a little different from a rawhide covered three. You might be able to remedy the dry spots by folding a small bath towel in half lengthwise and placing it on top of your pad to see if the slight additional thickness helps remedy the problem. Or add a second one. Just be sure the folded edges aren't under each other but an inch or so apart.
For what it's worth, I have a Corriente and most of the horses I ride are super wide, mutton withered horses and it fits them great. Even fits my 16.1 half draft boy.
I have really been eyeballing a Corriente saddle. A lot of the organizations around here give them away as trophy saddles and they always look very nice. I've been trying to think of someone I know well enough to ask if I could borrow one and see how well it fits.
The horse's body changes as it becomes more fit or less so. Also with age. Your saddle seems to be a fairly close fit. Don't use a pad any thicker than 1". The space at the back is part of the air channel. The angle behind the concho seems good. Just be sure the front concho, which denotes the front edge of the bars, is behind the edge of the scapula by a couple of fingers. The front edge of the skirt can sit on the scapula, just not the bars.
A different pad may help, but looking at those pictures without a pad the angle of the bars seems to be a little different then your horses shoulders. I also have a billy cook for my mare who is wide as a truck and it fits perfect! Best of luck in your saddle/pad hunt! :)