Permission to travel with a minor to Canada?

I applied for a visa to Travel to Canada but it was declined. I am originally form Botswana in Africa and I am in the USA with my 6 year old son. His father is back in Botswana. I was planning to spend Christmas in Canada but I was told to get a Notarized letter from his father. How do I go about it? Does he need to get it notarized in Botswana and send it? Please advise