I totally feel your frustration! We have one like this, too, and it has been a lot of trial and error figuring out what worked for us. It was a combination of things. It required a lot of consistency and more than a few confrontations with our sun, but this is the most successful either of our teenage boys have been, yet.
A lot of kids (according to the school counselor) have a hard time with the organization and self discipline they need when they enter middle school. So, we instituted the universal binder. It is a binder that goes to every class with them. We found one that even had an accordion folder in it so that there was a section for each class. You could use tabbed dividers with pockets, instead. In that binder went every note, every piece of homework, every handout. All of it. It came home with him every night. That way he always had the stuff he needed to study or do his work at home, and he always had his homework to turn in at school. That way we didn't hear,"I forgot it, I left it in my locker, I grabbed the wrong binder, I grabbed the wrong book" and so on. On Thursday, before binder check on Friday, he brought all of his binders home and trasferred everything into the appropriate binder. We made weeknights study nights. He gets no more than two hours of free time, and that doesn't include games, going to friends. He can get on the computer, talk on the phone, watch tv, but that's about it. All the other stuff is saved for the nights when he doesn't have school the next day.
We have a computer site called progress book that the school uses to help us keep track of his grades, upcoming assignments, missing assignments, and tardies. We check it every other day. If you don't have that, contact the teachers and tell them you would like an email every week of his grade and any missing work. Explain that if he has a grade lower that (what you deem acceptable) or any missing assignments he will be spending the weekend studying and making up the work. It doesn't matter if he will get credit for the late work or not. It is about the fact that a person in authority told him to do something and he is going to do as he's told and follow through with his responsibilities. Most teachers, when they know you are on board and are supporting their effort to teach your child, will go miles to work with you. If, in fact, he has an unacceptable grade or missing work, all of the stuff that you have saved for him to do on the weekend is GONE along with the normal stuff he would get to do. He will be studying and making up his work.
Lastly, we instituted a rewards system. For every month that they keep their grades up and have no missing assignments, they get a reward. Sometimes it is microsoft points for the x-box. Sometimes it is an itune giftcard for the ipod, or an application for their phone like a game or ringtones, or even a visa gift card for a shopping spree. Usually we keep it to $15 or $20.
I think it's important to stress that you are not doing this TO him, but FOR him. If he doesn't develop these habits now, what is he going to do when the work gets harder, the teachers step back more, or he is off to college and you are not there to check on him. You are preparing him for success.
Hope these ideas help in some way!