Have you tried riding him bareback? Does he buck at all then?
Has he been seen by a vet regarding the issue?
During the trail ride that ended with cinch sores, was he in pain then? Did he buck then? Did you give him adequate time off to heal the sores (aka at least 4-8 weeks)?
I suspect that he remembers the pain, and is bucking to avoid potential pain and get the saddle off his back. There is also the chance that he has nerve damage, and the cinch is actively causing him pain. If you use your hand to palpate the area, doe he act like he's in pain?
I'd have the vet out, have him thoroughly assessed, and see what your options are in the event he is actively in pain. If he is doing it because he simply remembers the pain and wants to avoid it before it starts - then you need a very qualified trainer to assess the situation.
I'd ride him bareback 100% of the time, until I had him assessed by the vet and by a good trainer (assuming he's not in pain bareback and shows no signs of being uncomfortable). I'd let the trainer re-introduce him to saddles, girths, etc.