i'm not a fan of the bg here, i think it's too dull...i'd incorporate some more pen shapes into the bg, and also the bg looks abnormal because his shadow is there, and because of perspective, it looks like the wall should recline going from left to right, however, it's 2D, so it looks off. it's got some potential tho.
it's not even necessarily the shadowing on the render, it's just that logically, the wall would regress from left to right, so the optics are off...this could maybe be corrected by adding some depth with black brushing on overlay layers or something. also feel free to comment on my sig a few threads below
if your gonna shadow the render bro ya gotta shadow all the shapes also or its flat what can work also is spot brushing in areas to break up depth perspectives..>=) and its all good