The answer to this question may reside in his writing. I recommend you reading a few Doors lyrics. I did, eons ago, and remember cryptic verses, with some vitalist energy. So yeah, maybe Nitzschean but not nihilist. But as I said, I read them a long time ago.
Edit: on second thought, I wonder if it makes sense to peg "larger than life" artists such as Brassens or Morisson to any particular school of philosophy. By definition, artists chose another way (art, poetry) to paint human nature than philosophers (analysis, refutation, synthesis). They cannot be bound by philosophers, they may well straddle several schools of philo, or reject them all. Artists are philosophical UFOs. They don't map neatly to philosophical schools. Why should they?
Poetry is polysemic. This is its strength. Philosophy on the other hand ought to be precise.
Poets must stir emotions; philosophers tend to shun them.
What kind of poet could be satisfied within the confines of analytic philosophy, postmodernism, or materialism? Wouldn't adherence to a particular message tend to make the verses lame?
I think it's in the second tome of the Buru Quartet, one of the most powerful political novels of the twentieth century, that a seasoned journalist explains to the young, aspirant writer Minke that he should not try and focus on any particular message -- that would make his prose sound preachy and defeat the purpose -- but should rather try and paint human nature in all its complexity and contradictions, and let the reader draw his own conclusion.
Pramoedya Ananta Toer did that very effectively with the Buru Quartet. It still makes me cry when I think of it, 20 years after reading...