From Chinese 揚子 (for "Yangzhou" directly or for a nearby ferry named "Yangzi") + 江 ("(major) river"). Its false etymology as "Son of the Ocean" stems from the homophonic misnomer 洋子江.
揚子江 (traditional, Pinyin Yángzǐ Jiāng, simplified 扬子江)
In Chinese, the name 長江 or 长江 is overwhelmingly more common. To the extent 揚子江 is employed in modern usage, it is typically translating names or phrases from foreign languages, where variations on it became standard for the entire river.