Dullness and a lighter coat in the saddle area?

it kinda sounds to me like the saddle doesn't fit quiet the way it should and is making rub marks. kinda hard to tell without seeing it myself, but if the hair there is rubbed and shorter looking, i would say the saddle doesn't fit. when you put the saddle on, everything should be touching evenly. i'm guessing you're using an english, when you girth up, try using the two outside strap for the girth on both sides of the horse. you could try using a treeless saddle too, alot easier to fit to the horse, and refit if his muscle changes. or if he gets wider, skinnier, so on.