I'd probably try the straight up approach first and ask him for that shit back. If he's not gonna be honest about and you know he's fronting get the locks changed or something (why haven't you done that already...?). Remember your in a tight spot because he KIND OF has the upper hand as he's not the one growing illegal shit, although you'd think his possession of the shit he stole would see him on the wrong side of the law too. Good luck dude!