I think you meant to say right rein for one of those? Horses usually have a stronger/ better side, just as we do.
Why not just keep it simple to start. Work on bending him to the inside a few strides, straightening him on the rein, then a counter bend off your outside leg for a few strides. Depending on his physical abilities right now this can be done at all three gaits.
Small bending circles at the walk really helped my boy when he was stuck in the left side of his neck. We'd do a few in one direction, then change and circle the other way, always focusing on him bending around your seat and leg throughout the whole body, not just the neck.
Good luck and stick with it! It may take your pony some time to develop that flexibility.
We are not human beings having a spiritual experience. We are spiritual beings having a human experience. -Pierre Teilhard de Chardin