How can we deal with middle school meltdowns?

My oldest child, 12 years old, is in his first year of middle school. Without warning, we have hit an academic brick wall. My once mostly straight A student is now failing half his classes. He's bombing tests and not turning in work. While I have done my absolute best to keep in contact with his teachers and have talked with him repeatedly about the importance of good grades and writing things down, I feel like I have led my little horse to water but he's refusing to drink.