Have you tried saddling with a loose cinch and walking him in a circle, then cinching more, walk again, cinch up?
I was taught not to cinch up tight in the beginning, just enough to keep it in place while you do other things. (I position the saddle first, then pick feet, then bridle). I always walk a horse after final cinching to test for freedom of movement. The multi-step cinching has eliminated the blowing up because I usually catch them off guard with the tightening step.
The 4finger rule is my guide on tightening. I make the cinch tight, but I should be able to get 4 fingers under it or it is too tight.
Also perhaps you need a stickier saddle pad for routine riding? I recommend the supracor especially in summer as they really do keep the horse’s back cooler, and they aren’t slippery.
Alois Podhajsky: “When I hear somebody talk about a horse being stupid, I figure it’s a sure sign that animal has outfoxed them. ...