Usually the way an owner does this is standing behind the horse and pull the tail slightly out and down and hold it, not a yank. What I'm referring to is a stretching exercise not a chiro manuever that rounds the back while the horse stands square, it is helpful with soreness to a degree. I'm not much into the untrained yanking personally, heck half the chiros out there scare me.
Is he turned out often? Without knowing if there is another underlying problem there are a few things to consider. Turn out often helps horses, when they roll and plan often they do more benefit to realign themselves than quite a few chiros I know. If you're like us though, our show horses unfortunately can't have as much turn out time as needed and they also don't recieve the proper amount of vitamin E that they recieve in green grasses.