Where did Jack's name come from???
Originally the name Jack was given to Ultraman Taro. The idea was dropped when they decided "family" to be the main theme of the series,and "Taro" means "the first child" in Japanese.
They wound up giving New Ultraman a name when they were making Ultraman Story, a movie focused on Taro and the Ultra family. The producer felt the character must have an actual name because in the movie the Ultra brothers would often address each other by their names, and it would be confusing using names like "New Ultraman" or "Ultraman II." They just picked Taro's unused name because.... you guessed it, because it's Taro's movie!