The 1858 Wright edition added the subject pronoun he to the conjoined predicate “said unto him”, probably accidentally. Elsewhere in the text, when referring to opening one’s mouth, conjoined predicates always lack a subject:
The last example, from Helaman 7:13, is very similar in phraseology to the example here in Alma 18:18. The 1908 RLDS edition removed the intrusive he here in Alma 18:18.
Summary: Maintain the original reading in Alma 18:18 without the subject pronoun he for the conjoined predicate “and said unto him”.