Answer:
This is way over my head, so I can only give you the only rational advice: Take him to a family doctor.
Dissociative Personality Disorder (used to be known as Multiple Personality Disorder) could be physically based, so a doctor's checkup is definitely a must. If there's nothing wrong with him physically, then he should be referred to a psychiatrist or a psychologist. The recovery rate for DPD nowadays is pretty good, so don't give up on him just yet.
Good luck
I'm not quite clear what your actual question is, but here is some info on Dissociative Identity Disorder (DID):
DID (MPD)
DID is the existence of two or more separate personalities or personality states within one person, with each alter having distinctly different ways of thinking, feeling, behaving and relating to the world and distinctly different memories, each part having amnesia for the other parts' memories. It is born from repeated and severe abuse and involves the defense mechanism of dissociation and generally develops before the age of 10 as children are far more likely to dissociate.
DID is often misdiagnosed and it is very common for a person to have had multiple different psychiatric diagnoses before it is definitively identified as DID. The symptoms frequently overlap with symptoms of schizophrenia, Bipolar Disorder, Depression, Anxiety Disorders (all), PTSD, other Dissociative Disorders and Somatoform Disorders as well as Borderline Personality. It requires extremely careful assessment and a high level of trust by the patient before alters reveal themselves. The diagnosis cannot be finalized before a therapist has actually made contact with another alter and observed the switch between alters.
The hallmark symptom is amnesia, which can be partial or complete depending on the level co-consciousness that exists between alters. Folks with the disorder describe the amnesia as "missing time" or blank periods, often daily or weekly, where they cannot account for their whereabouts or behavior. It is this amnesic barrier between parts that often leads to the most bizarre and distinctive signs and symptoms: not recognizing familiar people; not remembering highly significant events in their lives (like the birth of their first child, for example); finding purchases or articles of clothing/possessions, writings or drawings that they have no recollection of having bought or created. They are frequently accused of lying because they disavow their own behavior which is remembered by one part, while the amnestic part is completely unaware of it. Other unusual symptoms include: an exceptionally high tolerance for physical pain (they split off physical sensation which becomes encapsulated in one or several alters without others feeling it); not recognizing themselves in a mirror; using different names; having dramatically different skills and abilities that seem to be alternately present and then vanish (one alter may be able to drive a car while the sudden emergence of a child alter results in complete loss of this ability until the adult alter re-emerges); completely different opinions and behaviors (leading to the mislabeling of Bipolar or Borderline Personality.
Often communication across between separate alters takes place in the form of hearing voices, hence these folks frequently get misdiagnosed as schizophrenic. The key distinction here is whether the voices are experienced as coming from inside the person's head (DID) or outside one's head (Schizophrenia/Bipolar Disorder).
The separate identities develop in response to traumatic experiences which the child is unable to integrate and so they become "split off" from awareness and begin to take on a life of their own.
Folks with DID often self-injure, frequently a result of internal battles between persecutor alters and weaker alters and there are continual battles for control of the body and "time out" in the body between competing alters.
Symptoms of depression and anxiety are frequent and common and the picture is further complicated by the fact that one alter can meet all clinical criteria for Depression, while another part experiences no symptoms whatsoever. One part can be psychotic and experience no side effects from meds while another non-psychotic part has all the side effects and will stop taking meds. You can imagine that attempting to medicate such a disorder becomes an absolute nightmare.