I say write it to the best of your ability. When you're writing in a voice that's drastically different from your own, I think that you always have to edit for consistency later. You just have to do your best to ignore the flow of what you're putting down.. which, I know, is much easier said then done   ;)

It might help you to go the extra mile getting into your character's head before each writing session. Really try to be him while you're telling his story, because he wouldn't go back to see if what he was writing sounded like him or not. It might also help to tell yourself that you can go back and edit the day's work once you've reached your goal for the day.

I've found that timed writing helps me to focus on just getting words onto paper. Get yourself a little timer (there's lots available on the internet for free), set it for ten minutes and write as much as you can for those ten minutes. When the time is up, take a ten minute break to walk around, get a snack/drink, read your e-mail or check LJ. Repeat until you're done for the day.