ドラゴンリーグ

Tokio and his father Amon go to the soccer country of Elevenia where they see a parade of the strongest soccer team of that country, the Winners. Amon challenges the team captain Leon to a soccer battle and, when Amon loses, Leon turns him into a miniature dragon. Tokio sees the fight and decides to challenge against Leon so that Amon can become human again.
(Source: AnimeNFo)