Does your horse have any issues getting along with other horses? Possibly all the horses in the ring for an under saddle class make him nervous. He is a Thoroughbred, and an ex-racehorse. Possibly he sees all these horses in the ring as 'race competition'.
Ihave to add that I did use Quietex with a mare at a few shows, and it helped a lot. It helped ease her nerves, and helped me to give her a very good show experience. The last show I had taken her to she was great, didn't quietex her and she went around the course like a champ. Sometimes they just need that help to calm down. Once a horse gets away with something so many times, it becomes a 'habit'. Quite possibly since he's continued to do, now he knows he CAN do it. Quietex will help him quiet down and relax, and will help you be able to soften/supple him more without his tension in the way.
I am not an avid Quietex user, and prefer NOT to use it, but it is there, as a last resort. Just don't become dependent on it. I only use it until I feel the horse has gained enough confidence to do it on his own. Horses stress like humans do sometimes.