First of all, and most important, get a thorough neuropsychiatric evaluation. Do NOT go to a specialist--at least not yet. When you take a kid like that to a psychiatrist who specializes in childhood bipolar disorder, guess what diagnosis you'll get? Same when you take him to a specialist in ADHD.
My oldest daughter, now 11, had issues similar to your son's, and what I just told you was the single best piece of advice I got. Of course you're getting conflicting diagnoses--medicine is so unbelievably specialized these days that it is a rare doctor who can see outside his/her own specialized expertise.
Fortunately, you are near two terrific institutions for the kind of testing your son needs: Children's Hospital in Washington DC, and Kennedy Krieger Institute at Johns Hopkins in Baltimore. Both are superb. Google either, along with the words "pediatric neuropsychology" and you'll find what you need.
My daughter--whose meltdowns were so spectacular that she got us thrown out of a public building in WAshington, DC when she was only 18 months old--was eventually diagnosed with ADHD, and is doing well today. ADHD rarely occurs all by itself, though; there are usually other co-existing issues, like sensory integration problems. Either one, or both, can cause a kid to act like he or she is totally losing his marbles. A good occupational therapist can help if sensory issues are involved. Feel free to e-mail me offline if you need names. Believe me, I've been there. Good luck. T. Content