LOL Possession. Sorry. The funny fact about this is that it sounds like Insidious in the description but without the coma state or the red face.
Anyway!~
Like Ravinclaw said, if help isn't wanted, then it can't be helped. The most you can do is talk to him and try to get him to listen. Once, a Christan friend of mine was being messed with by a dark spirit and it took me only saying 'God is with you and will not let harm come to you. Just believe in his protection and imagine it as a warm blanket, wrapped around you' for her spiritual defenses to go up. It started to leave her alone and after a ritual I did on my own and beyond her knowledge, things were all ok.
From one median to another, most of anti-possession is purely determination. 'This will not get me cause I say it will not' and believing it, and you are generally in the clear. If you let it scare you, it scares you.
Perhaps the approach you should take is the psychological one and do some tests. See if he will consent through persuasion.
Him: There is nothing wrong with me!
You: then prove it to me, and take the tests!
Him: I don't have to!
You: is it cause you are scared that there will be something wrong?
Him: (whatever)
Take an approach such as that and if he is under the age of 18, drag him along via parents as previously stated. If all else fails, get the name of the shade, and make a puppet with arms and legs and a head of fabric; fill it with herbs for binding or whatever you wish, then wrap it with black ribbon, rope, shoelace, or whatever and as you wrap it, say 'I bind you (name) to this puppet, along with your power to prevent you from doing harm.' over and over and over. Have your covenant help (as mine did with this same ritual) and then put it in a box with seals on it to be dealt with later. If problems continued, means it's mental and you have wasted your time in worrying about the spiritual sense.
*shrug* Back to other thing I suppose.