Just needs to be a standard letter with 1-2 paragraphs, along the lines of "I hereby confirm that I will provide financial support for XXX for his/her studies in XXX" and then outline what his job is, his salary, how much your program costs and how much money he will be giving you. You need at least CAD$10,000 for every year that you are here just for living + tuition. It is better if you pay the first year of your program in advance, and he can say that too. Then end with that they can contact him is they have any questions. Have him sign it and attach all the supporting documents.