Yes, my bro was classic extreme ADHD - had every symptom in the book including problems with focus, but the focus issue wasn't the primary problem. Don't get hung up on the "superior" bit... although you just contradicted yourself a bit - creativity is a form of intellegence and yes, my brother has that too (his degree was in fine arts)."superior intelligence". This may not at all be in the formal, academic sense. He does seem bright and aware. Creative in his thinking. That's what you read about ADHD kids, though. Did/does your brother have ADHD?
You're right - you don't take the "superior" term to school, but you can take the "sensitive" one - and the "immature" one... explaining that, as part of the uneven development, some aspects are advanced while others lag - and you suspect that J's perception is advanced while his coping skills are not.
The other concept you might want to take to school is the term "executive functions". THIS is where J is struggling - the inhibit function, for example (self-control). This is a common problem in ADHD - and can also occur with other disorders too (Autism Spectrum Disorders (ASD) being one of those).
As far as the focus thing goes? He may be "inattentive" rather than "hyper" in class... K2 had a fantastic, absolutely top Kindergarten teacher - who did not believe that K2 was ADD. She is... over the top. She can be looking right at you and her mind is somewhere else... but she's bright enough to catch the edges of what the teacher is saying at the same time, and so does "fairly" well - but medications made a huge diff. So... just because the teacher doesn't see a focus issue doesn't mean there isn't one. If its there, it will show up later as the school work gets heavier.
Just some ideas...