What do we do about this behavior?

My stepson is a very intelligent 12 year old but lately he has decided he doesn't want to be responsible. He says he just wants to ignore every responsibility and have fun. He is now failing most classes, getting sent to the office constantly for not participating and doesn't realize that it's worse for him when he doesn't do his work.
We're trying to help him understand the benefit from doing his work as opposed to the negative consequences that arise when he does not do it. He will sit in his room for hours staring at his homework because he does not want to do it. He cries and refuses to take any responsibility and when questioned, will most time shut down to where he won't speak and hangs his head. He will sit down in PE class and turn his back to everyone and ignoring his teachers. He will be sent to the principals office but will not learn from this behavior. He has started being disrespectful to his father, myself and his teachers. He lies, covers and hides things from us. We've tried various punishments (no video games, no TV, etc), reward systems and point systems (all of which he helps create and seems excited about initially). We make things in writing so he can visually see his progress but nothing we seem to do makes him accept any responsibility. It's frustrating because there is effort and trying on our part, but there is zero effort on the child's. Do we just let him fail now? Or what do we do? I feel we've tried all we can think of without even the slightest turn-around. We need help and advice...

Thank you.

[Sandman2]

  Well, you haven't tried everything that you can do.  You have tried to do a lot of things - and the fact they haven't worked should be an indication that its time to try another avenue.
  His behaviors sound very much like a kid with ADD who has scraped by in elementary school - probably with B's and C's - because he is fairly intelligent.  About 5th grade the not being able to concentrate begins to really show up in areas like math.  It doesn't take too long for the child to become frustrated and just give up.  Sounds like that is the stage he is in now.  Basically, you don't except responsibility at this stage because you know that no matter how hard you try - it ain't gonna work.  
   Its sad, if he was hyper, this would have been picked up long ago.  But because he was a decent, smart kid just quietly getting by ( and maybe his parents had other things on their mind at the time) - it wasn't noticed.
  Of course, he may be just a lazy no good so and so. You deal with that type of individual in one way.  If he does have something like ADD - its a whole different ball game.
  My first suggestion is to get the book, "The ADD/ ADHD Answer book," by Susan Ashley, so you have a feeling for what might be going on.
   Second, do talk with your school about doing some testing.  They should be concerned by now also.  Unfortunately, ADD is harder to detect.  If you can afford it, go to a pediatric psychiatrist who specializes in AD/HD.  
   These are just the starting steps.  The book I recommended will be very helpful if indeed he does have this problem.  And, I should add, that if he does - it is very treatable.  If not - well hope that it is just a phase he is going through.
    I monitor the ADHD forum.  Please post over there if you have any more questions.  The posts get kind of lost over here.  Best wishes.