Nelson does the same thing, and I would end up doing alot of "bad habit" fixes to attempt to correc the issue, when in the long run I was doing it incorrectly.
For instance, when he would drop that inside shoulder, I would try to pick it back up with my inside rein.
Nelson is really bad, going to the left. Instead of us going around a bend in an arch ) it felt like we were going around at an angle \ - well, after riding with Dorothy Crowell *She's an Olympic Eventer* I found out, it isn't my horse, it is me. The result we are getting, is because he is trying to compensate for my lack of balance...even though it felt like I was balanced, I truely was not centered over him - so in turn, he had to go around like that, to "feel right".
So, what Dorothy had me do to correct it, was to over compensate with my upper body/shoulders by turning them overly much so to the inside. So, if I am going to the left, I want to overly exaggerate my shoulders by turning my upper body to the left as well.
Then, open up my inside rein, and ask with my inside leg, while supporting with my outside rein, to remain on his shoulder to prevent it from popping out.
Here is a video of me riding under Dorothy, maybe it'll help you out? I got some "breif" moments of correct movement, but listen to her, to grasp what to do...don't watch me...lol.