To get the best advice, you may need to provide more information. What has been done to address this problem so far? Does he have a diagnoses of ADHD? If so, are there co-existing conditions along with that? Or something else altogether?
Step number one is always to find out any underlying medical conditions, like ADHD, that contribute to the problem then work to find a treatment plan that works. From there it becomes possible to apply various therapeutic techniques to improve interest, motivation, and focus on school work.
In short, give us a little history and context.