Larissa,
Pdd-nos should allow you services under the category of "autism" if you wanted/needed to go that route.
In my state, pdd-nos falls under the ASD umbrella and so is categorized, educationally, as autism. This provides all children, regardless of label, the proper services.
I wouldn't take it so lightely. Early intervention is best. Yet, if your son is so high functioning and doesn't require any services right now, then I'd give it time. If some issues need to be addressed, they will unfold.
I do know that Aspergers is not diagnosed in a 3 1/4 year old and is usually reserved for older kids. My son is seven, and, frankly, I feel he would have been better served with a dx of Aspergers. High IQ, no lang. delays, social impairments (but not enough). He didn't fit profile exactly according to evaluator. Still not convinced, but he has access to services so it'll do for now. Most people that meet him would agree with AS that are familiar with AS especially as he gets older and talks more out of context about knowledge and interests.
As for growing out of pdd-nos, the clinician who diagnosed your son is misusing the diagnosis. Everything I've read suggests otherwise. If a neurological disorder does not exist, the diagnosis should not be given to a child. A wait and see approach is best in this case. Yet, I have heard of some kids outgrowing pdd-nos, but maybe it's because it wasn't really there to begin with and was given as a temporary dx so child could access services.
I'm merely speculating. I wonder about these issues--hence, the post.