During 1998 I had Dale Caswell, a researcher on the Book of Mormon critical text project, search for cases of potential pronominal mix-up between them and him in the Book of Mormon text. This instance in 2 Nephi 25:8 is one that he identified as a possible candidate for emendation. From a grammatical viewpoint, one would expect “and he that supposeth that they are not / unto him will I speak particularly”. We already know of cases where Oliver Cowdery initially mixed up Joseph Smith’s dictation of either them or him (for examples, see the discussion under 1 Nephi 10:18–19). Here in 2 Nephi 25:8, we do not have the original manuscript for the word them, but it probably read as them. It is very possible, however, that the original text read him and that Oliver misinterpreted Joseph’s pronunciation of him as them (given that both can be pronounced as /ßm/ in unstressed contexts), especially since he had just heard Joseph read off the pronoun they (“and he that supposeth that they are not … “).
In discussing this issue elsewhere (see under 1 Nephi 10:18–19 and 1 Nephi 17:48), I have argued that the original text allows pronominal switching between singular and plural, providing the pronoun is generic. And here in 2 Nephi 25:8, the pronoun he is generic, which means that the following them may be intended. For that reason, the critical text will accept the them, the earliest extant reading. Nonetheless, there is a distinct possibility here of scribal mishearing. One possible revision of the standard text would be to replace the them with him.
Summary: Maintain in 2 Nephi 25:8 the them of the earliest extant text, the printer’s manuscript (“and he that supposeth that they are not / unto them will I speak particularly”); the original text allows switching in number for generic pronouns; nonetheless, in this passage them may be a mishearing of an original him.