Start with his teachers and school counselor at the beginning of this school year. Ask them to work with your son and all of the other students in his class to help them plan projects to be completed over a period of time. The teachers can require an outline the first week, note cards the second, a rough draft the third, and the completed project the fourth week. Helping students learn to manage their time like this will assist them through high school, college, and on into their adult lives.
The school counselor may be able to include your son in a small group on good study skills. Many colleges now offer short courses for middle- and high-school students on how to develop study skills, and it is often motivating for the students to go visit a college campus for this class.
At home you may want to tie your son's love for music to academic motivation. Perhaps he could participate in private lessons or get sheet music or CDs in the weeks when his school work is turned in appropriately; if there is a week when the teacher calls to report that your son has not done his work, there is no lesson or CD.