Interestingly, as soon as you write it, it's copyrighted.
http://www.copyright.gov/circs/circ1.html#wwp "Copyright is secured automatically when the work is created, and a work is “created” when it is fixed in a copy or phonorecord for the first time."
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However, the obvious problem with -not- going and registering you copyrighted it, is proving that you wrote it, and that you wrote it first.
More than that however, is that, unless he was running a module of some sort (in which case, the idea would belong to the company/person who wrote the module), then the rights to the idea and the storyline would be considered to belong to him (providing he has notes and such, as almost all GMs do).
The greater problem than this would probably be the friendship though. I can't really give much advice on that, you'd be a bastard to rip his ideas, and it sounds like he's being a bastard by trying to change the work of which (it seems) you are being the primary writer.
It really sounds to me like this is a no win situation. You dump the project, and you risk the friendship (possibly, i don't know what type of person this guy is) and you waste a year of work. Dump him from the project, and you definately risk the friendship (I know that i would be extremely angry if such was to occur to me), and if you were to ever get published from this, there would almost undoubtedly be legal contestation, and even worse, he'd most likely have a fairly good case against you.
Really, I can't tell you what to do in this situation, my best advice would be to try to talk it out with him. Try to come to some agreement.
As a note, I don't have any legal training, so don't take my advice here as definate. This is just my understanding, since, as a writer I try to understand copyright law so that I don't get screwed over
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Now, just as one more note, I would say that this is why I try to avoid collaborative works, not that they don't work, I'm just a control freak. I want it to stay how I make it, and while I'll listen to other people's opinions, it doesn't mean I'll change it. I've actually done writing from a D&D campaign before, what I did however was to take my character (whom I created on my own, as I wanted him), and put him on his own, with my storyline, and with the Gods I created for that world (I help come up with some of the content of the world in the campaign, which i happen to guard ferociously as my own. Even if it might piss other players off, I'll claim it as my own just so that I can avoid circumstances such as this... and the DM is my best friend... hrm.)
Well, this is just my two cents... if you'll excuse me I think that I'll go try to find out when I became such a control-freak and when I became so paranoid. O_O