Thanks for the feedback everybody. I went back and broke up his fingernails more, added some more detail into his forearms, sculpted up some teeth for him, and a bit of minor detailing in other places.
With his hair I'm currently planning on doing a bunch of alpha planes and using what I have from the sculpt as the solid core - definitely want to make the hair bigger and scraggly, plus add those few stray bits coming up from his forehead. I also agree about adding a lot of stains and grunge to his clothing, I want to make him look like he's been sleeping in the back alleys of the city.
As for his shoes, well, they look like socks because that's what I was going for. I searched for a while when looking up reference to find any images from the comics where I could tell what he had on for shoes, and in the later issues and renditions of the character it looks like his has these long pointy socks on and I couldn't find any showing shoes... so he'll be wearing some old dirty brown socks on my version of the character.
I still need to add the shoulder spikes to his jacket and the handle on his jacket zipper, but I think that's about it. Anything else I'm forgetting or can improve on him?