He uses index, middle and ring finger.
He mostly uses two fingers to pluck 8th on one string, for example, and uses the third (ring) to dampen the next higher string and/or pluck a note there. This pattern changes around, depending on the bass line and the strings involved.
There are two basic "positions" - open and closed, which one he uses depends on whether the bass line is more one a single string or involves more string-skipping.
I haven't seen the video in a while, and it's quite hard to describe.
Here are two links to his right hand lessons:
http://www.garywillis.com/pages/lessons/rh1.html http://www.garywillis.com/pages/lessons/rh2.html