Don't know your original intent, but if the subject were placed further from the light source the falloff would be lessened. You can use a reflector to fill the shadowed side of the head and thusly separate subject from the bacjground.
Thanks for the feedback. I will definately remember that the next time. However, this was more of a candid shot. So, I wasn't quite prepared. I just really liked how the light from the window reflected off his face.
I find the composition interesting. In a lot of cases the falloff here and grain does not work, but I find this effect nice with this image.