Even if this kid is telling the truth and did not put a bag of weed in his bag, that only means that he is hanging out with people who have a bag of weed and who are shifty enough to hide it in this kid's bag. Either way, this is not the type of kid that you want you kid associating with.
Tell your DS that if he continue to hang out with this kid then everyone (classmates, teachers, counselors, principal,...) will believe that he also smokes dope. He will be guilty by association. And he will be labeled as a pot head and a bad kid or a troublemaker because of it.