Here the 1920 LDS edition changed the subject pronoun he to of him since an equivalent rephrasing of the first sentence is “but little should be required of him that hath but little”. The following parallel clause that begins with the prepositional phrase to him may have served as a model for the change to of him. Nonetheless, the original reading with he is perfectly understandable. This particular example of editing increases the formality of the Book of Mormon text but without any gain in comprehension.
Summary: Restore in Mosiah 18:27 the original reading: “and he that hath but little / but little should be required”; despite its technical ungrammaticality, the original text reads perfectly well here.