It would help him if you didn't lean forward while cantering.
Your heel is up because you aren't distributing the weight down the back fo your leg. It's stuck somewhere, perhaps at your knee or hip.
But your horse is more level, not as much on the forehand :)
I agree that your weight is"stuck" somewhere. IT's not comeing down through your leg. There is pinching happening somewhere, perhaps?
When you canter, listen to the 1. 2. 3. Feel of the rythm. Count it so that the the 3. Is the beat where the leading leg hits the ground. At this point, the horse will be in his most "downhill" position, with all the wait on the fore inside leg. You can help yourself by also thinking of YOUR weight as going even more down into your stirrups, with this downward beat. It's a visual helper to kind of think of going "down" with that downward beat. I also think of my pubic bone as going "down" , too. It helps me to stay "glued" to the saddle when the saddle is actually going downward . It you don't go downward with the saddle at the same speed, but through resistance in your body, you go downward slower, you end up being out of synch with the horse's up and down of the canter. Of course, if you are totally relaxed, this won't happen. But, for those of us, like me, who tense up at the canter, I found it helpful to actively think of almost "pushing" my pubic bone down into the saddle when it is "falling" with that third beat of the canter. Same with the soles of my feet into the stirrup. NOT bracing against the stirrup (which happens when you lock your knees) but kind of "sinking" the whole leg down, as if you wanted to drag the sole of your foot along the ground.