yup now there's a point what does he say when you ask him WHY ?
I would for sure see a doctor about this to make sure that there is nothing physically wrong with him. Also see if he can tell you why he is doing it.
You should try having a frank (not angry) conversation with him. I know I had issues with using the bathroom at night when I lived with my parents. I was TERRIFIED of the hallway going downstairs and didn't want to have to deal with that fear to go to the bathroom. I know it sounds crazy but that hallway still gives me the creeps to this day. I have no idea why. Anyway, it could be something like that. There is just NO telling until you discuss it with him.
PS make him scrub the closet out and disinfect it .
Certainly at his age this should not be going on and so yes you have to make him do without a privilege he wont want to lose ..like the Computer,phone or any tech item he cares about.It seems like he and the others have got away with it, so get tough on him ...