In the manga he is introduced using the Japanese pronoun for 'he' which is, 'kare' appears in Hiragana next to the kanji for 'he' or 'him'; "そして彼が (Soshite kare ga) Yamashita Satoru-kun da." This can be found in the original Japanese manga, Volume 2, page 86. Additionally, "Satoru" is traditionally a masculine name in Japan.
The Wikipedia article (in the Manga/Anime ring) does refer to Yamashita as a girl. However, this mistake was based on an English scanlation of the manga that has proliferated on the Internet. (Info taken from Myanimelist.net)
