I am a bass player, and I try to play like Phil (a tough order, but I try my best).
Like your bass player, I don't actually know much theory. When I started playing bass - maybe 12 years ago - I was undoubtedly pretty awful. What I DID learn though, was every damn scale I could. Then I applied my ear to what Phil was playing in a particular song and went with that. An as example, my band learnt Lost Sailor a few years back. Couldn't tell you about the WHY of something, but a little time listening to Phil and I now play an F# Phrygian mode over the first part. Just comes down to knowing the music and learning the scales all over the fretboard.
Normally I never like to 'learn' a bass line (after all, it's all about the improv, right?) but there are a few times when you just need to knuckle down and steal a Phil riff - Slipknot! being a prime example.
Other than that, playing Phil is enormous fun! Just make sure you get the intro to The Other One down perfect.....
Now you can all mod me down for not knowing all my music theory
