Thanks for the reply.
It seems more likely a case of Psychosis , that your son is suffering from as of now. The fact that he feels people want to hurt him, he talks to himself when no one is around, sees objects and images which really are not there, concedes of increase in sexual drive and feels other people can see his thoughts, all these features are suggestive of Psychosis , as in pure medical terms they are known as delusions of persecution, auditory and visual hallucinations, impairement of reality testing , illusions , and all these are hallmark characteristics of Psychosis.
Now , this Psychosis can be either due to Mania (Bipolar disorder) or Schizophrenia, for diagnosing the underlying psychiatry disorder, you need to consult a Psychiatrist, who will carry out a detailed mental status examination along with the general physical examination and will eventually come with a diagnosis.
As far as my experience goes i would consider the Psychosis due to Mania of Bipolar Disorder, due to the age of onset of his symptoms, which are in teens. But this can only be confirmed by a proper meeting with the psychiatrist in person .
So, i hope you take an appointment with a Psychiatrist for your son and get him evaluated. Please, this needs prompt attention as this psychiatry condition does have a bearing on one's life , but with proper medications and suport one can deal with it .
I Hope my answer serves your query according to your satisfaction.
Regards.... Dr. kaushik