How do I ask him to do take his shoulders or hind off track properly?
I hate to tell you this but, if you don't know how to do it, you need somebody experienced from the ground to show you how. If I explained it to you, there are so many variables, you could end up doing the wrong thing and get really good at it too! My best advice to you is to get an instructor that knows how to train a proper canter, among other skills, they can tell you when you're out of position when you think you are in the proper position, etc. I used to train horses for the public, it was not beneath me to haul the horse that I was being paid to train, to another trainer to have them show me how to do it properly when I was running into a problem. Best money I ever spent.