I'm glad you're keeping him on the heartworm preventative. I checked the heartworm incidence maps for Canada, and although Ontario is lower than many places in the US, it's still a moderate risk, and definitely the highest incidence in Canada. See:
Real-Time Map (306 cases of heartworms in Ontario in 2017, and 119 cases so far in 2018).
https://www.albertaanimalhealthsourc...rtworm-disease