From everything you have said, it is plain as day that school is boring for him. Also, if P.E. and art are the subjects he is still doing well at, then it seems obvious that he would rather be doing something than sitting being talked at, which could be what he is getting a lot of at the moment.

Of course there's not a simple answer, but I personally believe that your son is the one who needs to take responsibility for his learning - it won't be motivating if anyone else does it for him. For example, if he can't face the prospect of writing yet another boring paper, then he should take some alternative ideas to his teachers (and coming up with some of these is where you can help). If he's into the techy stuff, then he might rather make a PowerPoint (which is not only on most school computers, but you can also do some really cool stuff with if you have a little patience) or a film (which doesn't have to require expensive equipment; you can make some really powerful stuff with stills and free software like MovieMaker or Photostory). His teachers are probably as frustrated as you are by his failing when he shouldn't be and although some of them might say no, nothing ventured, nothing gained.

Good luck and hang in there. Just like Biciclista's sons, my brother was a bright yet mediocre school student who graduated university with an M.A. with first class honours.