hey
I think you need to darken the shadows, or add another darker shade todo so. I was also thinking that you might wanna make the lighting more direct also.
edit: Well I see the light as coming from the top left of the image down to the right, but there is no shadow on the shirt and neck to indicate this.
Umm, if your gonna blow it up so big then you will have to be really careful with your proportions, because any slight mistakes at this size could blow up to be fairly major ones.
editI wasn't gonna mention this cause I thought you would already know, but you should also properly light the shirt and neck.
another editHere is a non human stylised edit I quickly done, dunno if it would be of any help or not, hehe.
and anotherAfter seeing Helm's comment I thought I better quickly try make my edit more human and less stylised and what not. It probably wasn't a good idea for me todo such an edit, especially since you are doing a portrait. But yea, I'm certainly no expert, far far from it, and there are likey flaws in this edit also, maybe I even added more from my previous, so be cautious. I'd listen to those in the know.
cyas