Thread:Ryousha/@comment-4762852-20150930010409/@comment-11479355-20150930071433

Not exactly so. The definition of 長男 as seen on jisho.org says: Noun1. eldest son (may be the only son); first-born son * かれ 彼 . *He stood godfather to my first son. Eiji could be the eldest of few or just the only son. More likely to be the only son as Surprise Future and other related media make reference to only Eiji as a child of Shinnosuke, no one else.
 *  は
 *  わたし 私の
 *  ちょうなん 長男
 *  の
 *  なづけおや 名付け親
 *  であった
 * 