A calm, generalized chat with him about coming up with a solution may help you find the cause. Beginning middle-schoolers tend to have this problem as a group, especially if they have to change classrooms. Let's tackle the first possibility. Make sure your son has a folder (notebook) that has sections for each subject and at least one pocket folder in which to place assignments. For a couple of weeks, check that notebook with him each evening before bedtime to see if assignments are in the notebook. Encourage him to take the notebook to every class. Talk with the teacher(s) with him present and find out what they suggest.
Be patient and consistent and give him the responsibility of making the grade.