Well, I want to know if he has ever been OK to the right? But, it sure could just be behavioral; He could have made up his mind that he WILL NOT go that direction as I have seen that many times before.
But, I would want to do one more thing before I started schooling on him. I would take a handful of grain and 'lure' him to the right with no pressure on his lead. You should try this first to the left so you can observe any difference. I do this any time I suspect a physical problem with turning. A horse should follow a handful of grain or alfalfa around all the way to his ribs. After all, they are made to be able to bite at a fly or other bother on their ribs. If your horse cannot do this or tilts his head at a odd angle, then find a better Chiro.
If he has no trouble following a goody around to his side, then start re-schooling.
The first thing I will do is take a 12 - 14 foot lead. I will stand to the horse's left side and bring the lead around his right side, (with my arm up over his back), place it behind his hip and then stand on the left and pull it. He should unwind himself by following his nose to the right. I will do this 3 or 4 times before I go to the next step.
The next step is to saddle one with an old stock saddle, preferably with a back girth. It is not unheard of to have one fight it and fall down, but that can usually be avoided by doing it this way. Take a ring cut out of an old tire inner-tube. You just cut across the whole inner-tube to make a big rubber band. The narrower the ring is, the weaker the band is. Start out with one that is pretty narrow and work up to a wider, stiffer one.
Tie the rubber band around the back girth (with a slip knot) just above the buckle on the right side. Then, pull his head just slightly to the right and tie off a rein to that rubber band. I use a full cheek snaffle so that it cannot pull through the horse's mouth.
You do not want to tie it too short. You want the horse to be able to give a little more than is asked so that he can get relief.
You will need to make the horse move forward after he has decided it is OK to just stand with his head off to the right. Some horses will just tuck their chin to avoid circling to the right. That is OK; Just have patience and it will outlast and out-pull him. I have not seen one yet that did not go to following his nose to the right. Some get really mad, but they all give it up.
I really like to do this in a round-pen. After the horse has given in to that rein, I will move the horse around the pen along the rail until he trots and lopes with his head slightly to the inside. Then, I will step in front of him, make him stop, make him make a 360 degree tight circle to the right and then continue on to the right until I step in front of him again. DO NOT let his go the wrong way with his head to the outside. You can get a real good 'follow his nose' thing done when you do this maneuver. It teaches a horse to gather himself up, do a pretty nice 360 while giving to the inside rein and then continue with his nose where it should be.
If you use a rubber link in your rein, your horse should not throw a fit and fall over like they will if you just tie their head around. I have never had this method fail and have had horses that people swore there was no way to make them go that one direction.