Don't waste your time doing the old 'pop it in a sealed postmarked and dated envelope to send to yourself' routine as I read the other week that its not binding or likely to stand up in court. You could have a look at giving it a Creative Commons license maybe - I'm not sure how firm they get with theirs? www.creativecommons.org
legally it is yours and copyrighted as soon as it is finished. I would give him a lower quality copy, like mp3 or similar. If you have the higher quality copy, then it it obvious that it it your work.
MikeD: not sure how much this will help your current scenario, but here's a copyright tips article at ASCAP.
If you have already written the song...you automatically get the copyright. If its just an idea...you cannot copyright ideas...you can rewrite any song or story you want in your words.
Says who? This proves absolutely nothing in a court of law. Contact an IP lawyer. If you can't, have your friend sign a couple documents. The first would be a NDA. The second would be a document spelling out what you are giving, that it is yours, that the song/project was not a collaboration and was created solely from your own ideas. Set out any terms in this document as well. Make sure to clearly detail every aspect.