[Private]
He might seem more restless and jittery or have more trouble concentrating or sleeping. He would be unfocused or tired all the time or even sick. If that guy suddenly loses interest in his Criminology class or what he likes. Or you. It might mean something's up.
He may even get more defensive or irritable, especially if you ask him about the drug or whatever. I mean he's already told you about it, so that's a sure sign there if he suddenly starts clamming up when you show concern.