I am currently riding a 7 year old appendix Gelding for his owner on the weekends only. He is a pretty well put together horse and a great little jumper but we are having TREMENDOUS problems when we track right on the flat- even at the walk. He throws his nose to the rail, pops his inside shoulder in, and his haunches weave back and forth as if he is totally confused about where he should be going. He then proceeds to move off the rail and to the inside of the ring- this is at it's most serious when he is at the trot and when approaching corners.  To try to remedy this, I am: wearing blunt dressage spurs and giving him constant and strong inside leg pressure to try to push his shoulder back out and get his haunches on ONE path- "fiddling" with my inside rein to try to get his attention and to be able to see his inside eye lashes, planting my outside rein with a constant and very light contact on his mouth.  I am riding him in a simple d-ring snaffle and a german martingale (which is new for him as he is not used to ANY contact in his mouth- his ill-fitting bit used to hang in his mouth before I bought him a new bridle [it had been like this for three years]). According to his owner, who really just rides for pleasure (no trainer) and does some light jumping, "He has always been stiff to that side so I just usualy don't go that way because he seems frustrated and I just don't think he is ever going to be a good gressage horse. "  She has hired me to "fix him" but I can't get to the root of this habit/problem. Strangly enough, this one-sidedness almost completely disappears when we hack around over fences! Help!! 

Have you ruled out pain-related issues with his back/teeth/saddle/etc.?

Saddle fits well and no back pain apparent. I have looked at his teeth and they seem normal he hasn't seen the vet about it but I- could teeth cause an issue to one side only?

I'd have a chiropractor look at him.  Alignment issues don't always cause obvious pain.  There could also be problems in leg or foot joints that would cause him to try and avoid bending and bearing weight in particular direction.  My horse started out the same way over a year ago, and it was similar in that it was less apparent over fences.  It was a combination of back alignment issues and topline muscle making it difficult for him to bend and balance evenly.  A chiro + stretching + muscle building makes a big difference.

Once you have ruled out pain and all that jazz.. then you have to MAKE him go in that direction. As you said, his owner never worked him in that direction because "he seemed frustrated". The horse I ride in lessons is one of those steady bombproof, do anything you ask, anyone can ride types; but as soon as I ask him to work in his bad direction (by work I mean doing spirals and asking him to come through his back and onto contact) he bulges out his shoulder and tries to get out of turning.  What Im trying to say is, he doesnt have as much muscle on that side, and its harder for him.

When you ride him in that direction, open your inside rein (dont pull back, just open it wide) and pull his nose to the inside. At the same time use your inside leg to push him over to the rail. When he is straight (or at least doesnt have his nose sticking to the outside) tell him he's a good boy, and bring your hand back to where it should be. Do this over and over until he finally realizes, oh... its a lot easier to go around straight.

I understand a little of what you are going through, maybe not to that extent but my horses has consistent issues to the right as well.  She is much much stiffer and has a harder time balancing.  Have you tried doing basic dressage movements with him to the right, at the walk?  Shoulder in, traverse, ronverse, turn on the forehand, turn on the haunches, leg yielding?  Another thing to do that would work on the stiffness up front is to just stand in the middle of the arena and take the inside rein to your hip and pull his nose to your foot, when he stops spinning in circles and gives to you, just let go.  After i have warmed my horse up i work on this to the right.  She figured out pretty quick what i was asking and it has really helped in her suppling.

thanks so much for all of your good ideas and advice! i am working on turns on the forehand/haunches with him and he pretty mauch has that down- he's ok moving laterally at the walk which made me think it wasn't a joint/hoof issue. he also will gladly turn his nose to my foot at the halt. I have been using the open inside leading rein as Stepher had suggested in her post- i will def. keep working him and try to get this sorted out- i am perhaps a little impatient since the other two i ride are so supple. i will have to try to find a chiro. in my area and talk the owner into paying for it- i think it could be good for him!

couple of thoughts...

Teeth: if it feels as if he is locking his jaw on the 'bad' side rather than his neck it could be teeth. If the mouth stays soft however, you're probably ok.

I'm inclined to not be too quick to say OMG it must be pain if you say he doesn't behave like this in the field... it might just be a mixture of stiffness and just plain not wanting to because it is hard (stiffness) and he's never had to do it before. You can run your hand down his spine and see if he flinches, if not still get the chiro out (especially as you aren't paying!) but also persevere with him under saddle.

Few things to try...

You can bring your inside leg a little further forward to help 'cue' that inside shoulder as you go down the long side. Also try using a stick with a gentle tap on that shoulder to get it to move away from you back onto the rail.  

Lots of shoulder in is helpful as is turn on the forehand and turn about the forehand. Leg yield can be good as well in both directions but watch he doesn't just lead with the dodgy shoulder and fall in/out (depending on which rein you are on).  

As soon as he starts to fight you and set against you, I would imediately turn him onto a 20m or 15m circle... then ask him to go straight again. You have to be quick to feel his balance shift against you and catch him before he 'locks'.

Try a bit of lunging to see if he can bend without a rider on board.

Barring any physical problems. That said, sounds like this horse is hollow right and stiff left. You say he is not used to ANY contact in his mouth. Could this horse simply be uneducated in the bridle and you are asking too much too soon? No matter his age.

I would work with this horse on the ground first with long lines, walk, trot. Under saddle you say this seems to disappear hacking around over fences but you didnt state what gait.  Could this horse just not be "forward" unless hacking around over fences? Could also be that this horse does not enjoy stretching into the outside rein (right). Its hard to say as their are so many variables when riding. Could you post a video? Try not doing so much and keeping your hands quiet for any trust issues. Sounds like his ability is being overtaxed. Put him on the lounge line with side reins both directions for mostly trot work, no more then 20 minutes. Im assuming he is uneducated in the mouth and doesnt know the limit of the bit yet. Horses are asymetrical...our job is to encourage them to be symetrical ...IMHO.