My younger brother is also six, and is diagnosed with a Pervasive Developmental Disorder (Autism, not otherwise specified). One significant marker for AS is precocious speech - i.e. child speaks very early in life. Children with High Functioning Autism as opposed to Asperger Disorder, tend to develop speech significantly later than their peers. If your lad was quick to talk, it is highly likely that he has AS as opposed to HFA. Since he is so young, it is hard to tell. My brother was given the "Pervasive Developmental Disorder" label at the age of 4yrs, because it was considered premature to diagnose him with Asperger's.
Best of Luck
P.s. www.withyoueverystepoftheway is a wonderful place to chat with other parents of autistic children. Although the organisation that maintains the site is based in New Zealand, it has patrons from all over the world - i.e. U.S.A., U.K., Northern Europe, Australia, and so on and so forth. It is in forum format - like this site.