If the saddle isnt fitting correctly, and causing pain/discomfort, then he will for sure resent having the girth tightened. I would have him checked for a properly fitting saddle, AND, I would have a vet out and see if he might have stomach ulcers. That would cause pain and discomfort too.
Once you eliminate those possible causes, and you completely rule out pain, then I would slowly start re training him to have a girth tightened. I would start by removing the girth completely from off the saddle, and wrapping it around him. Any grumpy signs, pinned ears, trying to bite, etc. a firm no, and a slap. Once he is alright with the girth being wrapped around, attach it to the saddle again, and try wrapping it around. Again, any grumpy or sour signs, give him a slap, and a no. Finally, slowly, slowly, slowly start tightening the cinch. Tighten it a tad, and give him a minute. Tighten a tad, and give him a minute. Any naughty signs equal a slap and a no.