Then what? What difference does a diagnosis like Autism Spectrum Disorders (ASD) or Pervasive Developmental Disorder (PDD) make?
in my opinion... everything.
If that is what you are dealing with, you need to learn about it in absolute detail. Including from people who have it... people like Temple Grandin and John Elder Robinson. People on the spectrum are wired differently. TOTALLY differently. In order to parent someone on the spectrum, you need a whole different approach. You change YOU in order to change your child. It's a long, slow process, but it works.
Even if the kid doesn't meet diagnostic cutoff for an official diagnosis, just knowing they have traits along that line is a huge help because that means... you have to treat them as though they ARE on the spectrum.