he forsook. It is taken for granted that we know the details of 1 Kings 14:22-24.
all. Figure of speech Synecdoche (of Whole), put for the part (viz. the ten tribes).
Related Commentaries of 2 Chronicles 12:1
2 Chronicles 12:1
1 And it came to pass, when Rehoboam had established the kingdom, and had strengthened himself, he forsook the law of the LORD, and all Israel with him.