I have the exact same problem with my gelding Pro. His back is literally FLAT. It drives me absolutely crazy. I've tried sooo many different saddles and Pads none make even a slight difference. The ones with FQHB's and Extra wide trees only make the problem worse for Pro. A breast collar helps quite a bit. I use a collar, back cinch, my 5 star saddle pad, and of course a front cinch (roper works well). It seems to work the best....Also make sure your front cinch is pretty snug (I always have to tighten it just before we run, then I loosen it after) - more then on other horses.
This type of breast collar works best with Pro (it has the two straps on the end instead of just one). It really helps secure you saddle.