Hi M.:
I am writing to you as a occupational therapist who has worked with children like your son for many years. Although I can't give you insight from a parent's point of view, I can say that the earlier the better for special education/intervention. Not knowing your son, I don't know about his strengths and areas of need but I'd be happy to guide you in the right direction if you would like to chat.
Here are a few people who work with children with PDD NOS:
Many children benefit from chiropractic care as they work on diet & or allergies that can sometimes play a large part in a child's behaviors. Speech Pathologist can work on language (developing or enhancing one's comprehension, social skills, reception & pragmatics to name a few) An occupational therapist can focus on sensory integration (how your child interprets and responds to his senses (touch, taste, movement, body awareness and so on); as well as self care (getting dressed, brushing his teeth), play, fine & gross motor skills.
Many children receive services at an outpatient clinic or at their community school. Two great places to check out for support groups, educational seminars and much more information are The Minnesota Autism Center http://www.mnautism.org/ and Pacer Center http://www.pacer.org/
Therapy for Me in Eden Prairie is a fantastic clinic that has speech and occupational therapy services as well as a specialist working on RDI. http://www.therapyforme.net/
As I mentioned, I would be happy to chat with you more as I know there is a lot of information to take in.
Good luck. There are a lot of therapies, some that work fantastic and others that work for a select group of kids. I do believe that diet plays a very strong part of his diagnosis. In my opinion, I would look into the Gluten Free Casein Free Diet http://www.gfcfdiet.com/ as a starting point. There are also supplements that children take. Dr Wand at the Midwest Wellness Center is a place to look into. http://www.midwestwellness.com/about.html A holistic person, or as I mentioned a chiropractor would also be able to guide you.
Autism Speaks is a great website to find information http://www.autismspeaks.org/ They go through each type of therapy available, probably in more detail than what I did up above.
I hope this helps :)
C.