Are you sure he is a trotter and not a pacer?
If he is a trotter, you are going to have a much easier time in training. I would first recommend a trainer to help you. Secondly, I would train him to recognize voice commands when lunging. Eventually, by doing this, you can verbally ask for the canter when riding. However, since I get a sense of the fact that you are a beginner rider, I don't think a lot of progress will be made without someone's help. Good luck, though.