Here we have a good example of where Oliver Cowdery misheard him as them while taking down Joseph Smith’s dictation (although the original manuscript is not extant here). The 1837 edition corrected the text to the expected singular pronoun him. The reason for the misunderstanding is the identical pronunciation of pronouns him and them in casual speech (both are pronounced as /ßm/). For other examples of mix-ups between these two pronoun forms, see under 1 Nephi 10:18–19.
The use of him here in Ether 8:3 is supported by other cases in the book of Ether where the text refers to single individuals as “serving in captivity”:
Summary: The correct reading in Ether 8:3, according to the context, is “did make him serve in captivity”; him was misinterpreted as them by Oliver Cowdery when he took down Joseph Smith’s dictation, but the 1837 edition corrected the them to him (which is what the context requires).