If you search the archives here, you will find lots and lots of recommendations, but not many in that geographic area. (That's where I live, too, though, and driving for a diagnosis isn't a big deal. This is someone you will see every few months, max, probably, so it's okay if s/he isn't in your backyard.)
Many people see a developmental pediatrician for a thorough evaluation. The most commonly recommended are Dan Shapiro and Chuck Conlon in Bethesda. I think Dr. Shapiro is only taking younger children now, though. Dr. Polly Panitz is affiliated with Capital Area Pediatrics and you can see some people's experience with her here. A lot of people also like Kennedy Krieger in Baltimore.
Other options for diagnosis include psychiatrists and psychologists.
Finally, a good source for referrals might be your son's school. Ours helped us get in with someone excellent.
Good luck!