1) Get a new vocalist (helpful, eh?)
2) There are tablet's, (over the counter type things) the like's of Wayne Static (Static-X) take to loosen their vocal chords - although I'm not a huge fan of popping any kind of pill for anything other than legitimate reasons.
3) Get the guy to try taking vocal lessons to try and expand he's vocal range and abilities.
Ultimately, if he's natuarally got a "feminate" voice, there might not be much you can actually do, but it's worth exploring avenues.
On a side note: Placebo, The Bee Gee's and Boy George have all made a fortune out of such a voice...(perhaps not what you're going for, but if it work's, it works).
Don't worry too much about me, ignore me long enough and I'll go away.