I think ISTP. The way he thinks and talks about things in his music, and also the way he is in interviews (both the ones where he's quite shy and unsure, and ones where he's full of bravado), just really reminds me of both my ISTP friend and also my step-grandma says he reminds her of my step-dad, also ISTP, when he was in his teens and 20's. I think he's quite common for how that type gets if they get into rebellion in their teens in a big way.
I don't think he's an extravert at all though. I definitely think his dominant function is a judging one, and introverted, hence Ti. That only leaves S or N to sort out, and since he tends to rebut criticism with 'facts' both about himself and the critic, amongst other things like the general sorta stubborn free-spiritedness with that slightly grumpy edge (again, something I see in ISTP's I know), I'd plump for S, but with a fairly well developed N, because of the things he 'connects' in his lyrics, and generally brilliant wordsmithery!
Not a Feeler though, Feelers don't have the monopoly on passion (and indeed the 'choleric' temperament tends to be associated with NT), and though a lot of what he says is venting feelings, he does it in an analytical and semi-detached kinda way, and he says things that I don't think any Fi type would say out loud or in public. And since I've ruled out extraversion that's Fe out, too.