PDD comes under Autism Spectrum Disorders. It is Pervasive Developmental Disorder. It is basically an umbrella term of 5 specific disorders. It is mainly characterised by delays in the development of multiple basic functions, in other words, severe developmental delays. If your doctor suspects or is indicating that your child has PDD, it is very important that you consult a child specialist.
I have given you one source, but you can get a lot of information otherwise as well. All the best!

Some things you might want to look at.  How old is?  Is he verbal?  Did he develop at a normal pace?  I teach children with autism.  My unit is called the PDD unit.  Is there other behaviors that are going on?  Often times children that develop at a normal rate and have verbal skills that develop have aspergers.  Often times, these children fall through the cracks because it is so hard to put a finger on what is really going on.  Has the school recommended seeing the school psychologist?  There are lots of factors that might need to be looked at.  Good luck!  Keep being an advocate for your child, that is the MOST important!

You need to get yourself some expert opinions right away.  Contact the Montgomery County I. U.  Ask the school for their number.  Depending on the age of the child, they might direct you elsewhere. They only take younger children. They will come out and evaluate your child without it costing you anything.  That is the only tricky thing.  Getting help can be very expensive.  However, the law requires that no child be left behind.  Therefore, start there and see what other services you can get without having to pay.  I would also suggest you contact Children's Hospital of Philadelphia.  The wait may be over a year however to get in.  You do not want to put any more time on the clock without  getting your child some intervention.  Remember; you are your child's first doctor. Do not rely on the school district to make decisions for you. There is also a women named Wilma Dorman.  She is a very well respected Occupational Therapist.  The name of her practice is O.T.R. Inc.;  they are located on Germantown Pike in a business developement right across from the Pretzel FActory. (EastNorriton?) She would be another wonderful person to see and evaluate your child.  She would be a lot faster than CHOP and can set you in the right direction.  
Examples of some advice you may receive is that your child may need   to join an organized playgroup where they work on social skills and communication, or on following multiple step directions (or a number of other things)  they might require some home therapy for speech skills or Occupational Therapy.  It could be so many different things and it could also be so little. But you should get evaluations and seek advice even if it's just for your child to get implemented into an Individualized Education Plan  (IEP) which under the law every child is entitled to, whether they are gifted or in need of something just a little different to enable your child to succeed.
Good Luck; and get started now!  -your child is depending on it!