My son is 13. He has admitted to me that one of his friends smokes cigarettes and marijuana. Should we continue to allow them to spend time together? How can we make sure that this boy doesn't influence our son?
It's a good thing that your son feels comfortable enough to tell you something about a friend that he knows you don't want to hear! You should feel proud of him and of yourself for developing a good relationship.
Since you know this about this boy, you may want to casually check to see if this boy smells like smoke or looks stoned when he is in your home. You will also want to be able to control the time your son spends with him. That is - invite him to your house, invite him to go with your family to a game, to the beach, to a fast food restaurant....Let your son spend some limited other time with him too but continue to hug him when he gets home (to smell for smoke, etc. as well as to remind him that you love him).
Don't assume that this boy will influence your son, assume that your son will influence him - and tell your son that you have that much faith in him. At other times, reiterate your family's rules about smoking and drug use (not in relation to the other boy, but just in general).
This advice applies to all your son's friends now and in the future - at least through high school. It sounds like you're on the right track already! Enjoy your this time with your son!