I am not a psychologist nor psychiatrist. That being said, I can try to provide my take:
Atypical means NOT typical. So he may exhibit SOME symptoms of autism but not all. The catgegory PDD-nos (not otherwise specified) this is a catch-all diagnosis when some symptoms are there but not EXACTLY as indicated in the diagnostic criteria.
I'm quoting the DSM -IV here:
299.80 Pervasive Developmental Disorder Not Otherwise Specified (Including Atypical Autism)
This category should be used when there is a severe and pervasive impairment in the development of reciprocal social interaction or verbal and nonverbal communication skills, or when stereotyped behavior, interests, and activities are present, but the criteria are not met for a specific Pervasive Developmental Disorder, Schizophrenia, Schizotypal Personality Disorder, or Avoidant Personality Disorder. For example, this category includes atypical autism --- presentations that do not meet the criteria for Autistic Disorder because of late age of onset, atypical symptomatology, or subthreshold symptomatology, or all of these.
I think DSM-IV TR is the most recent version. The DSM (Diagnostic and Statistical Manual) is what psychiatrists use to diagnose conditions and disorders. . It is published by the American Psychiatric Association. Autistic spectrum and PDD disorders are around codes 299.00 to 299.80.
You may want to sugest to your consultant to update his/her information. Maybe this diagnosis may give you insurance problems later. I'm not sure but that the diagnosis atypical autism my not be currently recognized by the American Psychiaatric Association. You could ask what version of the DSM was used and and show you the classification criteria. It seems to me PDD-nos (299.80) is perhaps the a more accurate description and coding.
Here are the versions up to the DSM-IV:
DSM Versions I–IV, 1952–2000
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