Grammatically yes, it is a mistake (and a common lazy slang), but imagine if Eddie Vedder had sung "he doesn't know..." Not as punchy.

It is grammatically incorrect. This mistake that is made often, even among native speakers. So often, in fact, that nobody thinks twice about it when they hear it in a song. I'm guessing he did this to make the line fit better. Imagine, in the same part of the song, him saying "he doesn't know"....it doesn't flow as well.