Here we have a generic use of the singular man that takes the plural possessive pronoun their in the earliest text. In his editing for the 1837 edition, Joseph Smith replaced the plural their with the singular his, in agreement with the singular man. Elsewhere the text sometimes uses their with a singular generic word. For some examples involving the generic whoso(ever), see under 1 Nephi 17:48; for examples involving every man, see under 2 Nephi 29:11. The critical text will restore the original their here in Alma 1:17.
Summary: Restore the plural possessive their in Alma 1:17, the reading in the earliest textual sources; in the original text, the plural pronominal form frequently refers to a singular generic word.