If you are really interested in hurting someone there is no better way to do so than to inflict some sort of mental pain. Physical pain can be overcome. There is always a way around it, but mental pain is a different sort of animal.
Take whatever he loves most, break it and then give it back to him. It can be an educational experience. How does he respond to adversity. Does he accept the damaged goods? Does he try to repair the damage or does he discard it and go about his business.
When he reaches that place in which he can no longer feel the warmth of the sun upon his back he faces a choice. Does he allow the darkness to invade his soul or does he hold out hope for a brighter day.