Best thing to do is to first go see a doctor. See if any or all of this is related to a medical condition he just picked up. The doctor should be able to tell you if it is something physically effecting him or not (and also causing him to feel paranoid).
If the doctor can't come up with anything, I would go see a professional about it, someone who works with invocation, evocation, and banishing a lot. Physically go see this person with the victim, introduce them, and have the victim describe their symptoms. This professional should be able to handle it, or otherwise point you in the direction of someone who can. If you don't have a nearby local occult community to find your professional in, I would go see a Catholic priest and see what they can do for you. There are still a few of the Catholic priests who know how to perform exorcisms, and while you may not find one in your area, if your case is proven they may send someone to help.
I wouldn't take advice from people online whose reputations, knowledge, experience, and abilities you do not know. I wouldn't find similar cases and try to do what that person did to fix it. Each spirit is different from the other; fighting one might make it back off, while fighting another will simply make it more violent against you. Cases of real, uncontrolled possession should be taken care of by people who know about it, not by teenagers who think magic is cool.