You should not bend over. My sensei always reminds us that we should be like puppets and have our heads suspended from a string from the ceiling. If anything, bending your knees or sinking into your hips should be enough to keep him down.
As far as what David said above me, I agree. My sensei always tells us to imagine both our fingers are linked via a string. When he lifts his arm up to attack, my arm should go up too to catch under the elbow. The reason he tells us to imagine the string is tied at the fingers is to remind us to lead with our fingers.