While it certainly won't hurt anything to have your son assessed for ADHD, I would ensure that if you do the person doing the assessment is well versed in GT issues. GT kids are often misdiagnosed otherwise as ADHD when the issue is being bored.
I think it's also important to note, he's an 8 year old boy, that's the nature of young energetic boys for heaven's sake. As you've mentioned, he respects authority when he's instructed, he's doing good work, this isn't an issue that is degrading much of anything it appears. A lot of teachers aren't very good at taking enthusiastic energy and directing it in a positive manner. Discussing ways to do that would seem a good start, it appears you've already started that. Now the same needs to be discussed with his teacher. See the link below for a start.
http://teachtrainlove.com/20-teaching-tips-high-energy-students/
