He probably feels more comfortable cantering on the other leg, out of soreness of balance.
Make sure you're sitting totally balanced in the saddle, and since he's only just got his right lead going, stick on a circle for a while until he builds up confidence. Stay sitting on your inside seat bone, and give him the inside rein. This will make it extremely difficult for him to change, if your weight is to the inside. One of the aids for a flying change is a shift in your weight and change of outside rein, so make sure your keep these aids clear as day.