Being able to concentrate on an interesting book does not rule out ADD/ADHD. Having a hard time concentrating on verbal instructions could be related to problems with attention, hearing, or auditory processing (a separate issue from hearing). Being in his own world could be a sign of attention problems or it could be a sign that what is going on in his head really is a lot more interesting than what is going on outside it, which I'd bet is true with that IQ, or it could be both. Giftedness does not rule out the idea that some symptoms or behaviors are due to ADD, and ADD does not rule out that some behaviors are the result of giftedness.
There is a book called "Misdiagnosis and Dual Diagnosis of Gifted Children" that may help you figure all this out.
I also hear that Ed Amend is a great person to help evaluate these sorts of issues in profoundly gifted children, good enough that it might be worth a family vacation in his vicinity.
I've sent you a PM, Gracew (flashing envelope).