Nobunaga Kyousoukyoku

From MangaHelpers:

History classes have never interested Saburo. He never saw the point of learning something that happened in the past, let alone a few hundred years ago. But when he falls off a wall, he travels through a wormhole and ends up in 16th century ancient Japan. He meets a very young Oda Nobunaga, one of the most influential figures in Japanese history, who turns out to be identical in appearance as Saburo. The real Nobunaga, sick of the castle life and wanting to travel, orders Saburo to take his place. Without knowing how to get back to his time, Saburo tries to make sure history isn’t rewritten for the infamous Oda Nobunaga. [vyc]

Won the 57th Shogakukan Manga Award for shounen.