Shounen Yo is the first ending song of Kamen Rider Hibiki.
The song describes the first half of the series, especially the character relationship between Hibiki and Asumu and also how the latter started off the series as a wimp but gradually grew into a person who was able to stand his own ground.
The term ‘Shounen’ was also used by Hibiki when he addresses Asumu, instead of calling him by his name.