The original text of the Book of Mormon allows for either singular or plural interpretations for the generic pronoun whoso(ever). Here in Moroni 7:47, the earliest text has the plural form them in reference to the preceding whoso. The 1920 LDS edition emended the plural them to him. It is worth noting that the them here in Moroni 7:47 could be an error for him. Such an error could have occurred in 𝓞 as Oliver Cowdery took down Joseph Smith’s dictation since in colloquial speech both them and him are pronounced identically as /ßm/. And as explained under 1 Nephi 10:18–19, there were mix-ups of these two pronouns in 𝓞. Despite this argument, however, there are enough instances in the earliest text of whoso(ever) taking plural pronouns to argue that them, the earliest reading here in Moroni 7:47, should be restored. For further discussion of this point (plus an extensive listing of examples), see under 1 Nephi 17:48.
Summary: Restore in Moroni 7:47 the them in “it shall be well with them”, the reading of the earliest textual sources; although them could be a mishearing for him, there is clear evidence elsewhere in the text that the generic pronoun whoso(ever) can take either singular or plural pronouns.