Also, going into winter it might help to try a back on track back pad or something to help his back stay really warm. Cold-backed and girthiness seem to be linked to some degree because I think it is tied to tension and anxiety. Warm mucles are happy muscles.
I also never girth a girthy horse tied. I put it on one and one, move off of concrete and out into the open, one and two, walk 30 feet, raise it to two and two, etc. The last girth check is done from the tack following the walk warm up. I also walk a few circles in hand and do some turn on the forehands where I really encourage the back hind leg to cross under. My mare was not 100% but it did make a big difference.