Part of your discussion with your son should include a reminder that it is far better to wait to use any type of drug (that's why you wait until 18 to drink alcohol up there in the north eh) until the brain has finished developing, which is why you feel it is safer for an adult to use cannabis than a child.
As an adult, what you choose to do in private is more your own business than a minor, who is still being looked after by his parents.

Although if your son knew that his mother would be upset, and that you were hiding it from her, too, you've unfortunately set a poor example of communication that you will now have to think about and improve by taking the initiative to talk more openly to the whole family about what goes on in the house, from pot smoking to illegally downloading media.