You can't get him for impersonating a police officer because you dont have that phonecall recorded so there is no evidence, im sorry because that would have been good. The best way to fuck him over in my opinion, is to put on some gloves and search his room for the weed and in the process trash it. He can't call the police and use you guys because his only proof it was you would land him straight in jail. This douche is just trying to scare you, and you shouldn't be. I guess a possible idea would be just to go to the police and inform them that this kids a dealer, it wont do much, but they might keep an eye on him (thats really not that great of an idea). Maybe you could just have your blackbelt friend kick the shit out of him, and make him give you what he owes?