To help your horse improve his canter, you will need to go back to basics to improve his frame - how he carries himself. In your video, he is in an inverted frame in all 3 gaits. That means his head and neck are up and his back is dropped. In this frame, he cannot bring his hindquarters well underneath himself.
He is also counter bent meaning his body is bent to the left when he is going to the right (and vice versa). As a result, he is unbalanced and falling in to the circle.
To perform at his best, the horse needs to work with his back at least level. This frame allows the horse to create impulsion by engaging his hindquarters as they reach well underneath his body. His hindquarters are driving his forward movement. His head and neck can then work as the balancing mechanism they are built to be. The further the hind legs reach under the body, the more the back naturally lifts. The horse can move with elegance and ease through all transitions and in all gaits even when carrying a rider.
You need to go back to basics with your horse and get him working in a long and low frame first. This will lengthen his tight back and neck muscles and start to strengthen his weak hindquarters. As his suppleness and muscle strength improve, the next stage is to bring him into a level frame.
Going back to basics means that you begin at the walk and development good walk/halt/walk transitions and changes of direction without any inverting (high headed frame). When those are consistent, move on to walk/trot transitions - again focusing on keeping your horse in a level or low frame. Only when you have developed consistency and your horse has built up the strength in his hindquarters and back muscles do you begin work on the canter. This takes time and it will help you if you can work with an experienced coach/trainer. Find one who does not use gadgets to "fix" problems. Your riding and knowledge will also improve.
Take a look at my blog
for more information on how your horse's shape affects the quality of his performance.