Here the original manuscript has the singular “unto his word”. In copying from 𝓞 to 𝓟, Oliver Cowdery initially wrote the plural words but then immediately crossed out the plural s, thus making 𝓟 agree with 𝓞. (There is no apparent change in ink flow for this correction.) Nonetheless, the plural is expected, which led the 1830 typesetter to set the plural words. In this instance, the error was not caught. The 1858 Wright edition accidentally, it would seem, reverted to the singular word. All the RLDS editions have continued with the singular.
Under 1 Nephi 16:24, it was noted that usually “his words” rather than “his word” is used to refer to a person’s word. On the other hand, “his word” usually refers to the Lord’s word, but there is textual evidence for “his word” referring to a person’s word(s):
Thus the singular reading of 𝓞 in 1 Nephi 17:22 (“his word”) should be accepted. In each case, we determine the correct reading, word or words, according to the earliest textual sources.
Summary: Maintain the singular “his word” in 1 Nephi 17:22, the reading in 𝓞 and the corrected reading in 𝓟; the singular noun word is sometimes used to refer to a person’s word(s).