manga - Why is the Kanji 「太」read 「ふとし]?

I found this one from a title of a manga チャンピオン太

As my understanding, it should be read just as「ふと」.

Answer

So-called i-adjectives in Modern Japanese used to end with -shi in Classical period. Some boy's names still retain those old forms, such as たかし, さとし, つよし, やすし, ひろし or ふとし (conversely, they wouldn't name their boys in Modern adjective forms like たかい, さとい, つよい etc).

There's one more thing that Japanese names often contain some "unordinary" kun'yomi of kanji that might beyond your imagination. They also derives from obsolete readings in olden times now mostly confined within personal or proper names (名乗【なの】り). For example:

たかし: 高, 喬, 崇, 貴, 隆, 尭, 敬, ...