Why would an 18 year old student urinate in empty water bottles and save them in his room?  We have addressed this issue three times, and he is aware the awkwardness.  Could there be a medical problem?  Is he doing something that he shouldnt be doing?
Degree understand your concerns went through your details. I suggest you not to worry much. There definitely is something he worries of. Some negative thought or false information must have crept into his mind and that could be the reason for this behavior. If you talk to his friends, you shall know. If you pry, there are supposed to be other problems related to this one. I sincerely think he has Obsessive Compulsive Disorder (OCD). Do consult a psychologist for expert help. If you require more of my help in this aspect, Please post a direct question to me in this website. Make sure that you include every minute details possible. I shall prescribe some psychotherapy techniques which should help you cure your condition further. Hope this answers your query. Available for further clarifications. Good luck.