A video would be helpful. However there are some good suggestions here regarding body position and rider crookedness affecting the horse.
A few other things to work on are your outside aids, and a few exercises. First of all, you must always ensure that the outside leg is slightly further back than the inside leg and that the horse responds to it in the corners by bending his haunches in and around.
The most helpful exercise for this is to trot all the way around the school, and put 3 10-12 meter circles on each longside, focusing on bending the haunches around such that the hind hooves track into the front hoof prints. Then do it in the canter with 12-15 meter circles.
Other things to do are leg yield in travers in canter, counter leg yield in canter (ie yield right in right lead canter) and spirals in and out with trot canter, and canter trot transitions.
Basically, you want to get him used to bending more around your inside leg, by using your outside aids.
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