It's the age. My fifteen year old is a straight A student, well behaved and an over achiever but when he was 9 he didn't turn his homework in, he didn't care about school and we worried so much that this would continue into Jr high and high school.
Continue to stay on top of it. stay consistent with the consequences and enlist the help of teachers in knowing what he has due and have them also do consequences with him.
My son grew out of it and is a wonderful sstudent now. Hopefully yours will be the same
He could need a supplement experts agree that all children benefit from a multiplevitamin-mineral supplement with antioxidants Vit C and Vit B Complex, check his diet as nutrition counts,eliminate some common allergic foods like dairy and wheat for a month to see if that helps. It could be a phase does he get enough rest and sleep, and at the other end does he play vigorous sports and have fun, ask his teacher how he does in class.