"Were you given an outline?"
Middle school and high school students are frequently provided with outlines for writing tasks. These can be very helpful, not just for them, but for you as you lend support. They help direct your thinking toward the requirements of the task.
If your child wasn’t given an outline, tell him to ask the teacher for an outline or for a general format he can follow. If it’s too late in the game for that, try a grade-appropriate format that would work, such as the popular five-paragraph essay format.