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Is he sound? You need to rule out any injuries first. Seems like he might be off on his front or rear left leg. If not, he may have a pulled muscle on his right side, or along his spine. I would carefully massage along his spine and right side to see if he has any sensitivity. Call your vet if he winces at any point or if you feel a pulse in any of these areas.

If he is simply the victim of bad habit or asymmetrical confirmation, you should be able to feel it even when he is walking. Dressage, dressage and more dressage is your answer. Use some lateral exercises (small circles, serpentines, leg yields, half passes) to teach him balance in both directions. After a few weeks of this, you should be able to see an improvement in both leads.

As a last resort, I would use draw reins to try to teach him balance. Even at a walk, these will force him to carry himself properly. Start slowly as these will use muscles he is not used to and you could injure him if you progress too quickly.

He may have had some kind of an injury at one time and has gotten into the habit of using his right lead so it is harder for him to do it to the left. Sounds you are doing it right the right way of asking and sitting.

I have had some one sided horses and the best way I have found is to not let them gallop of the easy lead. Always ask for the hard lead and only do the hard lead for many days and often they will seem to limber up and it will get easire for them if you do it enough. Good luck!!

Try cantering circles. It makes the lead easier to control and it lengthens their strides because they are forced to have the rite lead and to lean in.