"Usually, we try not to use underlighting." That philosophy is from a not-very-creative point of view. I only sometimes agree with it, but in this case, I think you could just use a white reflector right below his knees.
That's beside the point. What you need to do, if you want everything exposed correctly, is move the flash away. On the third shot, it looks like the pillar was only about a foot (or less) away from the flash, and his face is more like 4 or 5 feet away. Thus, the pillar will have considerably more light on it. Move the flash farther away, so that everything is about 10 feet away. You may have to jack up the power on the flash unit, though. If the flash doesn't put out enough power to do that, well, that is the downside about strobist-y stuff. You always have to compensate for something!
Keep shooting!