I looked at your pictures the saddle one is to small to tell anything (about the fit). Your horse needs to gain some weight and muscle. His back is pretty weak. Those white marks up by his wither are because of how ill the saddle is fitting (im not sure if its from you or the previous owner because you said you just go him) Personally I would try the diamond wool if you don't want to spend a bunch of money, and get some more weight on him. They are nice quality for the price. $100 bucks isn't anything if you are talking about making your horse more comfortable!
Poor fitting saddle marks are from previous owner, I've only had him a week or so and am working on putting some weight on him already and toning up. I'll have to look into that pad I think