Y'know, I honestly think he doesn't even think about it. I mean, he writes with a pencil and paper. It's not like he does what most people do these days and moves shit around on a text document.
I think he just has a natural gift for it. A rhyme starts putting itself together in his head, and he siphons it through to the paper. I don't think he has to spend too much time cleaning up unnecessary syllables, reaching out for more multi's to add, etc. It just happens.
EDIT: Also keep in mind that the guy's been doing it for almost 20 years. Writing music is a dance he has mastered.