Okay... first thing's first. A psychiatrist and psychologist would approach the diagnostic tree the same way, although the psychiatrist would be moving ahead more directly, and with no room for any therapeutic interventions - i.e., his interview would last about 15-20 minutes, and be solely delivered for the express purpose of figuring out which medication to prescribe. Now I will say this, unless the kid was already in therapy (apparently not, given your scenario), he would probably be sent to a psychiatrist first - especially because the authorities seem to have been involved. With an act of violence like that, he would have been ordered to undergo a psychiatric evaluation, regardless of whether he was ever adjudicated for the attack - in other words, his lawyer (at the very least) would have arranged a psychiatric evaluation to make him appear more sympathetic in the eyes of the judge.
But onto the diagnostic process...
FIRST SCENARIO (the attack): Following this one, my questions would flow through BOTH an examination of acute stress (bullying, dropping grades, extreme parental pressure for academic performance, losing a girlfriend, drugs, suicidal ideations) AND a look at a family history - this would be to assess a possible genetic link to a psychological malady (i.e., looking at family substance abuse, depression, stress, history of violent behavior). Lastly, I would take a close look at (a) diet and especially over-use of caffeine, and (b) sleep patterns and potential for frying himself through not sleeping well or not sleeping enough. Now, I recognize that you covered a few of these in your scenario, but a shrink would not know that, and (frankly) would want to ask the questions himself anyway.
Here are the diagnoses that would be floating around in my head: substance abuse, even if acute; a bipolar episode (this would require some history of similar behaviors, even if they are not to this extreme); a dissociative disorder (this would be the primary course of my inquiry - a history of trauma would almost have to be present to create this mental landscape, but this is easily the most apparent); brief psychotic disorder (pretty rare in teenagers, but on the table because the scenario fits).
SECOND SCENARIO: With the addition of the vivid dreams and the apparent withdrawal into a world that does not parallel reality, I would be more aggressive at going after trauma, either in his history or in the present tense. So I would put more pressure on inquiry regarding his relationship with his caretakers, any potential for abuse at the hands of others during his developmental years... or, perhaps being victimized right now by a bully at school or maybe sexually abused at the hands of an adult. Because schizophrenia does not "come on" like this (I can tell you more about this if you are interested), and truly does not affect school-aged kids in this manner, my mind would not be moving in that direction.
Diagnostically, I would look more closely at a dissociative disorder - given your storyline, this would not be accurate within the context of the story, but as the therapist, I would not be completely privy to everything that a reader would be aware of - therefore, I would pursue this avenue, at least for the time being. Too, I would look more closely at whether there could be some sort of neurological damage... extending sleep patterns could imply something going wrong with the sleep center of the brain (the reticular activation formation), and the more vivid dreams could also be the result of an insult. So, I would refer for a neurological exam to rule out a potential blow to the head, tumor, or degenerative disease.
THIRD SCENARIO: Now that our hero is beginning to tear away from reality (at least, the reality that is perceived by those around him) - I would be more convinced that we are either dealing with a neurological issue, some sort of psychotic disorder (again, very rare, but I would not, in good conscience, be able to professionally ignore the mounting data) - like schizophrenia (which would require a prodromal phase), a brief psychotic disorder (depending upon the time line), some type of extreme dissociative disorder (which would require abuse of emotional trauma), or maybe a personality disorder such as schizotypal personality disorder. Bipolarity would be out of the question, and by now he would have been drug tested at least twice, so substance abuse would have been ruled out. There is not really another choice that would cover all of the symptoms, so I would narrow my focus to the handful of diagnoses listed in this paragraph.
Hope this helps... let me know if there is anything else, or if you need clarification. I love this kind of stuff! If you are satisfied, please hit "Accept." That is the only way I can receive credit for my answer. Bonuses are also appreciated. Thanks-