Basically, he's a pretty monotone guy, but there'll be lines where you have to exaggerate him a bit. Also, it'd be nice if the actor were able to improvise a bit, as a good bit of his lines are inner monologues.

*UPDATE*

I know I said monotone voice, but I don't think that was the right word for what I meant. I wanted the voice actor to sound, "Jaded," to the events going on around him, and no matter how crazy a situation gets he just seems to have a, "meh," attitude. Sorry on the miscommunication!