Unfortunately, if your son does not have Canadian citizenship, he will find it extremely difficult to enter a Canadian medical school. For example, UBC, my medical school, only accepts Canadian citizens.
You should check all the web-sites for Canadian medical schools to see whether being a Canadian citizen is required before applying. You can find links to all of these medical schools using the Links page of my website: www.geocities.com/mdpremie
Simply go to the Links section, and go to Sean Winters' page. There, you will find links to all 16 Canadian medical schools. Hope this helps, and best of luck with your grandson.