OK, so that is absolutely typical of people with ADHD - really smart but doesn't perform well due to inattention. If you get diagnosed as an adult, that is one of the first things they ask about - how well you did in school. I was a perfect student grade-wise but that didn't mean I didn't have attention problems!

You'll definitely watch this very carefully. You don't want him getting behind on the actual content of the classroom learning because that gets compounded over time and can also cause emotional problems if he's always behind.

It might be worth seeing if there are other ways for him to learn the content without doing worksheets or really repetitive exercises - not only is that hard with ADHD, but it's really really hard if you're too darn smart for the work you're being asked to do! My guess is that the design of classwork and homework follows a traditional and uninspired model of teaching that simply doesn't work well for people with different ways of learning - and that's not something that can be easily addressed by anyone other than the teacher (but good luck on getting them to adopt different educational techniques to support the full range of learning styles...)

If there are other alternate arrangements available, he might do better (without a dx/medication) in a non-traditional learning environment. Montessori schools, charter schools, and some public schools with special programs can be a huge improvement for people who don't do well with worksheets and lectures. Unfortunately, that's probably an issue that's going to be a challenge for a long time, so it might really be worth considering non-traditional alternatives and looking up if there's anything available in your area - enrichment programs and study skills workshops may be really valuable when he gets a bit older.