Have you looked into neurofeedback? My eg/pg 2e son (6.5-yr-old) was born with severe plagiocephaly - severe flattening to the right hemisphere to his brain - which is seen as a cosmetic issue that doesn't affect cognitive or behavioral domains by the medical profession, though this is completely erroneous. My son's symptoms also mimic ADHD and he was misdiagnosed with it too.
Everyone wondered why we just didn't medicate him, but I refused because I wasn't entirely convinced that he had ADHD. I just had a gut feeling about it.
We started neurofeedback and have since 'discovered' that he has excessive delta brain waves and this is NOT ADHD, but describes some of what you've had with your son and what you've been going through. And yes, medication would not have helped at all.
Neurofeedback may completely eliminate the need for medication and may be the solution to your issue (emphasis on may). In any event, it might be worth investigating since it does sound like you're facing a somewhat similar brain-based disorder. My DS (6.5-yr-old) too has high verbal ability and low working memory, similar struggles, and issues of being overwhelmed like your son.
I hope brain mapping with neurofeedback will become much more accessible and available to parents - and eliminate the misdiagnoses and medication. It's much more objective and definitive than a clinical observation or paper-pen tests and the guessing games. Either a child has high theta brain waves to confirm an ADHD diagnosis or they don't. Instead, too many of us get the run around and spend years and a lot of time and energy on a detective hunt for answers.
