Well, since the sun is in front of him, the back of him isn't going to be in the light, so that'll give you an idea of where the light will be, which will be mostly in the front. Also, don't just use variations of black or red or whatever the base color is for shading or highlights, use a mixture. Since it's a sunset there's going to be reds, yellows, oranges, pinks, mostly, with a few blues and purples near the top. Also look for other pictures for reference.