I would like to see a talented therapist, one who has worked with kids like your son, serve as both a understanding advocate for your son and a go-between between his teachers and him, meeting with all of them and devising ways to reach this bright boy. I have seen many schools respond to situations like this by creating more challenging schoolwork for gifted kids, including homework that was more than rote, busy work. Often, one adult who truly believes and appreciates a child is what is needed to rescue them from their own sense of powerlessness.
Let me know what progress your son is making after you follow up on some of my suggestions.