My thought is that this may be a combination of trying to "pass" socially (this is one of the most challenging grades socially, for NT and GT alike), and not seeing the point of maintaining excellence academically. Prior to fourth grade, it sounds like he received positive adult feedback in school for his academic excellence. Then fourth grade happened, and that disappeared. Now the spell has been broken for pleasing teachers with his high-quality daily work, at the same time that peer social dynamics ramped up a level in complexity, with everyone simultaneously realizing that one's social status can be quite precarious, potentially with severe penalties for sticking your head up in any way (with the possible exception of athletic or videogaming excellence). Their skills are rudimentary, compared to older adolescents, so all of the jockeying for social position, seeking peer approval, avoiding being in the out-group, etc. takes a particularly brutal form. Socially-savvy outliers not infrequently take the path of flying under the radar as much as possible, which, in this case, may mean lowering his performance (at least superficially--hence mostly in mechanics) to blend with his classmates.
And I would agree with Portia. Whether with words or actions, he currently is communicating that it is not fine. It may be that adults can help him develop a solution to his unspoken concern, and it may be that they cannot, but we will not know if he doesn't voice the nature of the concern.
