One of the best ways to tell if a saddle fits properly is to put it on and exercise your horse (preferably by riding). Ride long enough for the horse to sweat. Take the saddle and saddle pad off. Look at his back. If you see any areas where there is no sweat, that is where the saddle is too tight and is effectively pinching him so he's not producing sweat there. You can try adding some extra padding (if you see no sweat areas, typically on either side of the withers). Sometimes just adding an extra saddle pad will help.
I did notice in the last post you had that his saddle was not sitting down on his back, this could indicate that it is indeed to tight at the withers.
edit to add pic
Try the wet saddle pad test and see if you get dry marks.