Prioritizing is something many adults and children have trouble mastering. Don't expect your child to automatically know how to prioritize homework assignments or his free time. But during the tween years he should be working on improving his prioritizing skills.
Help your child keep a calendar of daily to dos as well as long term assignments. Also, children who have trouble getting everything done may have too much on their plates. Consider taking a look at your child's commitments, to make sure he's not over scheduled.