What grade/age? If he's spending a great deal of his day trying to go along with a curriculum he's already mastered, that can really cause some issues (behavioral). Put that with the ADHD and it's not surprising there's a struggle.
Neuroscience research suggests that the longer one has to maintain control in the face of a challenge (ex. trying to listen to a lesson for the fifth time when you already understand), the less able that person is to maintain control for the next event.
Are there any project-based or Montessori schools that might be an option? Flexibility is key, imo/experience.
If his parents are still learning about giftedness, I would suggest Jim Delisle's books. Having tried numerous authors and books, this is my favorite: Parenting Gifted Kids: Tips for Raising Happy and Successful Children.
