Watch out. He can be killed with any sword on Easy difficulty, but once you up the difficulty to normal or higher, you need a diamond sword or slendersword to kill him.
If you manage to kill him, he may drop a child soul, which you can use to craftslenderarmor. The slender mask gives you a higher chance of avoiding nasty effects he causes, such as blindness or nausea. Also, the static effect made famous by the Slender game for PC will slow you down if you look at him.