I can understand your concern about your friend's behavior, which may be due to the cannabis or other drugs that you state he uses, psychiatric illnesses (such as schizophrenia or bipolar disorder), or medical disorders (such as closed head injury or seizure disorder resulting from the car accident). He will benefit from a thorough medical and psychiatric evaluation, to obtain a clear diagnosis. If you are concerned that he is a danger to himself and/or others, and he refuses to seek help despite your encouragement, you may go to court and petition him for involuntary evaluation and treatment, or contact your local police.
You can't get a diagnoses on the web. He might be showing signs of psychoses. Can you call his parents or sibling and let them know? He really needs an evaluation.