This verse starts with an explicit he
(= Azariah). It means that his father started to subdue Edom, and he, his son Azariah, finished this work by making Elath into a strong(er) harbour city in Judah; see also 2 Kings 14:7 and the explanation of it. Another name for Azariah is Uzziah, and under that name we encounter him often in the Chronicles and the prophets.
22 He built Elath and restored it to Judah, after the king slept with his fathers.