Cheap way - burn a CD of just your tracks without vocals. Burn another one of just his vox track(s), and a complete one. Lock all three in a safe somewhere. Supposedly, the minute you "Make a recording" of a song, for all intents and purposes it's copywrited.
Better way - Send for copywrite forms (or download them) from the US copywrite site (don't remember the URL, search google or AskJeeves for copywrite) and copywrite the song as "Words by wierdo, Music by CoolGuy" (guess whose side I'm On?) BTW, this (I think) will also require separate CD's or cassettes, and probably two separate copywrite applications.
Best way - I didn't catch what area you're from, but try to find an ENTERTAINMENT lawyer and ask specifically about separate copywrite. I've never heard of this being done, only the separate credit approach. (Doesn't mean it hasn't already beed done though, just that I am unaware of it.) This last way is gonna cost more, but if you are both concerned that much about separation, it would be the way to go. If you're not in New York or LA or Nashville, good luck looking in the Yellow Pages... Steve