You seem pretty stable, which is good, and your horse is really obedient. It looks like he'd keep cantering forever if you asked him! :)
Like you said, your lower leg should be a bit back. To help you tell, you should be able to draw a pretty straight line through your shoulders, hips, and heel.
Your horse seems a bit stiff. His canter is also pretty slow. To fix this, lift your hands up a bit higher and urge him on a bit with your leg. Then try to move your hands back and forth with the movement of the horse's head. A bigger canter should also help to bring him off the forehand.