Some kids with disabilities, particularly if they are on the autism spectrum, don't test well and it doesn't mean anything about their ability. Pervasive Developmental Disorder (PDD), by the way, means he falls into the autism spectrum. My son's actual diagnosis when we first tested him was Pervasive Developmental Disorder (PDD)-not otherwise specified (pervasive developmental disability not otherwise specified). He has been upped to high functioning autism or Aspergers (they're not sure and I don't care. The label gets him the help he needs).
My son consistently tested 75 in school. Then he went to a neuropsychologist at 11 and tested at 110. Now he is sixteen, doing great, always on the honor roll is pretty much mainstreamed and, although he has differences, is doing MUCH better than I ever dreamed. I personally don't trust IQ tests for "differently wired" children. However, I think you should get some help for him at school. If my son hadn't had a lot of intervention, he would not be where he is today. Good luck 
