would you please take a few more photos, with saddle on without a blanket, showing more of the front angle of the tree bars(front edge of saddle), and how it does/doesn't follow the line of the shoulder. take a photo , if possible, of front of saddle (to show wither clearance) with someone IN saddle.
side view, cincehd up , no blanket, side view uncinched.
I think it might be a bit too wide in front, however, given your situation, with expecting him to muscle up, this may be tolerable, with some shims in the pad to lift it up off the wither a bit. However, if the angle is too flat (too wide at bottom of bar,) then shimming it will not change that and would only aggravate it being too tight at top, too loose at bottom.