Middle school can be a frustrating transition for tweens as they take on more and more academic responsibility. As frustrating as it can be, it's so important that your child develop certain academic skills, such as prioritizing and keeping up with assignments without your help. He'll need these skill in high school (and beyond) so now's the time to develop them.
Of course, it can be difficult for parents to know when to take charge ("Do your homework now!") and when to back off. If you're unsure about how involved you should be, try this. Give your child the chance to tackle his work on his own without your interference. If he does it, give him even more space to continue to develop his skill set. If he doesn't, you know you need to remain involved a little longer -- but continue to encourage him to take the lead.