Thanks for the great replies.
I think I may have a way to deal with this niggling issue. One of the maddening things about this is that the voice I'm using does actually fit the character--it's just that that voice isn't all that interesting on its own.
So the action of this story is pretty fast-paced; it's not supposed to be particularly deep or anything. Those portions and much of the dialogue don't require a lot of voice. It's mostly the introspective moments that suffer. Very recently, I wrote such a section that was better than all the previous attempts (corroborated by my trusty go-to writing friend), rough though it was.
After sleeping on that development for a few days, I think the way to tackle this will be through relying content more than on voice. The recent bit I wrote had stronger opinions expressed in it, and felt more genuine than the previous stuff. A handful more of these types of moments could go a long way toward fixing this.
That, and I also like the idea of punching up some the other characters around him. (No, I don't mean hitting them in the face.)
It may not be perfect, but I think I can live with it.