Just wondering if it would help to keep the girth lose at first when you first cinch him while you brush your horse and bridle him up? Dont tighten right away. Once you get the other things done then go back and tighten. Are you maybe tightening to much? Just some thoughts.
My mare was cinchy and would even try to come around to bite when tightening so I tried a different approach as I suggested and it has worked for me.