Start with the subject ' letter of invitation from *your name*'
Then write about your details like name,citizenship, your status and address in Canada. In the second paragraph, write about your mother's details, the duration with dates when you want your mother to visit and stay with you and the reason why you want him to visit you. In the third paragraph, write about your education,source of income, your current employment and conclude with the name of documents you are attaching to prove your identity and that you can support him financially while he is Canada. You can enlist documents like copy of passport, copy of driving licence, copy of employment letter and payslips for last 6 months.
And i think you should just write ' To whom it may concern'.