Hello! Please remember that my response is for information only, we are not establishing a therapeutic relationship.
His behavior does sound strange. However, if you knew him for a long time and his behavior only changed after you told him that you were interested in him, then there is a possibility that there is no mental disorder.
I describe a few other possibilities, but keep in mind that no one can give him a definitive diagnosis over the internet .
First --there is no longer a "Multiple Personality Disorder," the name has been changed to "Dissociative Identity Disorder."
Here are the diagnostic criteria for DID:
A. The presence of two or more distinct identities or personality states (each with its own relatively enduring pattern of perceiving, relating to, and thinking about the environment and self).
B. At least two of these identities or personality states recurrently take control of the person's behavior.
C. Inability to recall important personal information that is too extensive to be explained by ordinary forgetfulness.
D. The disturbance is not due to the direct physiological effects of a substance (e.g., blackouts or chaotic behavior during Alcohol Intoxication) or a general medical condition (e.g., complex partial seizures).
So --I can see why you are thinking along those lines, but DID is extreme, the person does behave like a completely different person.
His behavior sounds like he has some social or communication issues to me. His going by your house when he thinks you don't know it sounds like he has some interest in you, but is afraid to deal with you directly. The acting like he can't be "bothered" could be the result of a lot of things --including anxiety, shyness, or a lack of social skills (especially if stressed) or something more severe, like Asperger's. The "snapping" out could be dissociation (does not have to be DID) or the result of just being overwhelmed.
Here are the criteria for Asperger's:
[The following is from Diagnostic and Statistical Manual of Mental Disorders: DSM IV]
(I) Qualitative impairment in social interaction, as manifested by at least two of the following:
(A) marked impairments in the use of multiple nonverbal behaviors such as eye-to-eye gaze, facial expression, body posture, and gestures to regulate social interaction
(B) failure to develop peer relationships appropriate to developmental level
(C) a lack of spontaneous seeking to share enjoyment, interest or achievements with other people, (e.g.. by a lack of showing, bringing, or pointing out objects of interest to other people)
(D) lack of social or emotional reciprocity
(II) Restricted repetitive & stereotyped patterns of behavior, interests and activities, as manifested by at least one of the following:
(A) encompassing preoccupation with one or more stereotyped and restricted patterns of interest that is abnormal either in intensity or focus
(B) apparently inflexible adherence to specific, nonfunctional routines or rituals
(C) stereotyped and repetitive motor mannerisms (e.g. hand or finger flapping or twisting, or complex whole-body movements)
(D) persistent preoccupation with parts of objects
(III) The disturbance causes clinically significant impairments in social, occupational, or other important areas of functioning.
(IV) There is no clinically significant general delay in language (E.G. single words used by age 2 years, communicative phrases used by age 3 years)
(V) There is no clinically significant delay in cognitive development or in the development of age-appropriate self help skills, adaptive behavior (other than in social interaction) and curiosity about the environment in childhood.
(VI) Criteria are not met for another specific Pervasive Developmental Disorder or Schizophrenia."
The impotence could be either physical or psychologically based, and the friend may or may not be a source of reliable information!
Right now it seems like your ability to communicate with him is extremely limited. If he cannot even acknowledge whether or not he is interested in a relationship (at age 60), it will be difficult to discuss anything of a serious nature.
Please feel free to follow up with me.