CAPD is a langague processing disorder, and Asperger syndrome is an Autistic spectrum disorder. It is quite possible that he has elements of both. Central Audio Procssing Disorder was the diagnosis dujour about 10 years ago, and many children with social differences, language issues, fine motor delays, gross motor akwardness, were diagnosed with CAPD, this was my 18 year old aspie's first diagnosis, followed by ADHD+a whole slew of langague, motor and emotional issues, then ADHD w/autistic features, and finally Asperger Syndrome at age 11. Your son may follow a path that is equally confusing, and the diagnosis may evolve, or you may have a diagnosis that makes sense from the beginging. If they are looking at both, he may fall into that gray area where he is not a "classic" presentation for any one diagnosis. If you find yourself in that spot, there are some important things to remember.
I know that it may not seem comforting but let me say that we have learned that it does not matter so much in a clinical setting what diagnosis he carries. It may matter for school purposes (although if he has any qualifying IDEA diagnosis and needs special education, it should not) However, our experience is that our daughter's treatment plan did not change one iota once she was finaly diagnosed with an ASD. If he has CAPD or an ASD, you will treat the medical symptoms he has and provide therapy for the needs he presents with. All therapy is provided based on specific needs, and needs are identified based on standard evaluations, not diagnosis; diagnosis does not limit the type or kind of therapy or intervention (a very important point to remember for writing IEP's.)
He may have elements common to both disorders in his treatment plan, and he may even have both disorders.
I guess I am trying to say, it does not matter so much in the long run, let his needs be the guiding force, both for treatment, and for questions, and try not to focus on his diagnosis, because that may change, or be confusing, or be inconclusive. As questions about your son, not what you suspect he has. It may feel wrong, and I know you want desperately to know what it is, but you will probably get appropriate priveate treatment without a diagnosis. The only real obstical you will have is with schools, so try to pin the evaluator down to get a diagnosis that qualifies him for IDEA services, if he needs those. CAPD may not, and an ASD will be a qualifier (if he also needs special education.)
Good luck with his evaluation. Get therapy and treatment for everything you see, and don't delay.
M.