Well, it does sound like you've done due diligence on looking for a 2E reason for his performance in school. I'm assuming you've had his hearing checked, as well as vision? If so, I'd probably move on to thinking about whether what your DS is doing works for him. Is he underachieving because he cannot achieve or chooses not to? Does he need extra help because he is doing poorly on tests, hasn't mastered the materiel, has bad teachers, is bullied, doesn't want to do the work, doesn't know how to study, doesn't see the point of expending extra effort to move from a C to an A? There are so many reasons for grades to not be top notch--only some of which are under your DS's control. Assuming you have access to his detailed grades, I'd take a hard look at where he is "failing". Tests high, homework low? Participation? Good on participation and homework, but failing the tests? Is is only in one subject? He's losing points somewhere, and figuring out where can help you decide how to best help him. Once you've figured out what aspect is the weakest, I'd ask him about it. Maybe he knows that he struggles with some aspect of school and does not know that everyone doesn't struggle with the same thing. Maybe he thinks you already know about it and don't need to be told. If you can narrow it down, you can tease it apart and help him succeed by targeting that weakness. Gifted or not, learning how to succeed in school is, IMO, a process. Moreover, it is a process that often needs to be scaffolded so that the child gains the skills that will help him throughout his lifetime. IMO, there is a risk that if you do nothing and the school is content with his "middle of the road" achievement level, your DS may assume that this is all he is capable of.
Reading this over, it occurs to me that your may also want to explore whether your DS learns well and eagerly in other settings --perhaps music lessons, chess club, or robotics club? This could indicate that providing more interesting, challenging courses, like the gifted program, would help.
Just to sound like total helicopter parent, I'd also be aware that you have only 1-2 years to get this straightened out before middle school. IME, middle school shines a bright light on any weakness and it can be hard to change a trajectory at that point.
