If the horse's back swayed, or just lacking topline? If you have a typical TB with a high wither and weak back, you will find it fairly difficult to get a saddle that fits. I have found Kieffer's to be a good choice, as they offer a cut back pommel.
As far as building up topline, the horse is only 3 so there is only so much you can do. Muscle will build as he gets stronger and fitter - with correct, consisten work. So under saddle, make sure he is going forward and using his back, not travelling in an inverted frame, but not with his head jammed down by jiggling your reins either (thats a good way to create undermuscle in the neck!).
Hill work will certainly help - but again, make sure he is going forward and swinging his back. Dragging himself up on his front legs won't do anything.
You can try lunging over small ground poles or cavaletti - Set them up on a 20m circle, you can start with a ground pole on each 1/4 of the circle, and build to 3 poles in one section and a cavaletti on the other side... and eventually a cavaletti on each 1/4 circle. Lunging over cavaletti is great for encouraging the horse to pick its legs up and forward, which stretching the topline.