I'm definitely more a writer than an artist, but if you would allow me some suggestions? :)

I think really playing up the light source of the moon, almost to an extreme point, might give it that real dark vibe you're after - Darkening his face, perhaps to where you can only see his eyes and/or the reflection of the glass, and darkening the rest of the room considerably might give it a good bit more dismality.

Moving from that though, I do like what you have here. As always, your masterful expressions shine through and you can definitely feel that something weighs heavily on his mind. I do love watching your work! ♥

Agreeing with Anathema there, lighting would actually the biggest factor in providing that 'grungy' sort of dark feeling you'd get from an image like this

best resources fall along the lines of 'high contrast lighting' or any sort of image like this, with light being forced through a window in a dark interior space. Doing so would really give it that -push-, i'd think! 'u'

It ended up with too cute an edge.

I quite like this one because I can see it's tagged digital but I had to look closely to confirm that for I was tricked into thinking it was skilled traditional art. I really like the colouring style you used here.