Albert Barnes Commentary 2 Kings 18:6

"For he clave to Jehovah; he departed not from following him, but kept his commandments, which Jehovah commanded Moses." — 2 Kings 18:6 (ASV)

Other good kings, like Solomon, Jehoshaphat, Joash, and Amaziah, had fallen away in their later years. Hezekiah remained firm to the last. The phrase cleaving to God is frequent in Deuteronomy but rare elsewhere.