"probably ADHD" would set off alarm bells, especially if you see different behavior in every other setting. ADHD should be diagnosed by a licensed professional - and your doubts being voiced should motivate the professional to rule out other issues that can have ADHD-like traits.
I had my son tested by a neuropsych in 2nd grade and again this summer before 7th grade - and both times ADHD was ruled out. That didn't change that fact that he spaced out in class, but it meant it wasn't due to ADHD.
Is there a gifted program at your son's school? If so, perhaps the IQ results will allow for some transition to more challenging and appropriate material.
