Although it sounds mean, what your horse is doing is dangerous to you, so give this a try (my Aunt Kathy used to have to do this to her horse): With your girth ready to tighten, kick your knee hard into his ribs just behind the girth to knock the breath out of him, and at the same time, complete the tighten-up.
Of course this is assuming your horse is not going to retaliate. If he does, time for a respect lesson in the round pen! Horses are like human children, so give them love when good, discipline when bad.
"Ride fearlessly, but think carefully"