I think you need to search for some references, that doesn't seem like correct form to draw a bow. If I'm correct, the bow needs to go higher, so the arrow is more in line with the face and there needs to be chest expansion, as he draws the muscles in his back together (which I know is the correct way to store the muscle energy). Basically, think of it like he should be bringing his shoulder blades as much together as he can, causing his chest to stretch out. I also thinking you usually don't bend your legs in the way you represent it.