Maybe school is oppressive him. Dr. Aldort says that being always a "good boy" can be dangerous, as the child lives for pleasing others. Your grandson may be going through self-discovery and doesn't not want to please others any more.
This is a good thing because he is much better off being himself and not a compliant clone. Some kids wake up in high-school which is too late. They feel caught in having to live up to expectations and end up with drugs, depressions or suicide.
So encourage your grandson to share his feelings and support him in wanting to pave his own path.
When I was a child, at about the same age I also had enough of being good. This was the greatest thing and my mother's support helped me be strong, study well what really interested me, and pass the grade on the rest. She loved me unconditionally of my achievement.
The boy must know he is worthy and loved no matter what his grades us.
Some helpful reading:
Unconditional Parenting
Raising Our Children, Raising Ourselves
Punished by Rewards
