Not necessarily about covid. Young people have a notorioualy hard time being approved because they have so few ties to their home country. No job, no property, no savings, no spouse or children waiting in their home country. Not sure of your son has an internstional travel record to countries like UK, Australia, US, etc. It will seem like an unusual time to apply given that travel restrictions are lilely to be in place for a very long time. So will the 14 days of quarantine. Parents and spouses applying are understandable given they are exempt from travel restrictions. CIC is unlikely to refuse but your son will need a strong application. The fact that you have 2 other refusals make you seem pretty desperate to visit Canada. It is still unclear what the purpose of your visit is