I have no kids but believe that honesty is the only way to go. Be blunt or brutal but, be honest. Tell him what is up and the consequenses for boosting anymore of your stash.
If it was me, I would say "smoke your weed but don't smoke mine or I will break your fingers. But do not let anyone know that I know you smoke or I will take away your privilage until you are old enough by law {18} to do as you will."