I have a child who was like yours - not doing well in school, very gifted, not understanding why it mattered that he was performing below his potential etc. He has no learning issues or deficiencies when tested. We started after schooling him to see if challenging him academically made him care about learning - and it did, he has progressed remarkably ever since. We also put him in music lessons, chess classes, robotics club etc and he has great interest, focus and progress.
So, we changed schools a few times until we found one where he is challenged, is learning something new to him every day, has peers who are equals and he does not need to "dumb down" to blend in - he is in a gifted class now with kids who are very driven and capable - and he is competitive now and cares about his grades because he wants to stay in the gifted group and poor performance might mean going to the general ed class.
So, as brilliantcp says, try to challenge him in non-school settings to see how he responds and if he is interested, then it could be a lack of challenge in school. Good luck.
