There's a large possibility that it used to belonged to his grandfather. Remember how he found his grandfather's notebooks and a 足球 ball when he was helping Atsuko cleaning up for New 年 when he was a kid? Maybe he found it among his grandfather's soccer-related stuff too.
posted 一年多以前
Oh, God! I think episode 121 showed how Endou got his headband! Around 分钟 6-8.