'Rashi' #1, Radak #1: It was not like his latter days, which is he sinned with Bas Sheva and counted Yisrael.

'Rashi' #2, Radak #2: It refers to the kings after David; they sinned at the end of their lives. Shlomo's wives veered his heart, and he built Bamos to Kemush and Molech, the gods of Mo'av and Amon. It says about Rechav'am "when his kingship was established and he was strengthened, he abandoned Toras Hashem" (12:1). It says about Aviyah ¹ "his heart was not complete with Hashem Elokav like the heart of his father David" (Melachim I, 15:3). Asa did not rely on Hashem against Aram, nor when he fell ill. Rather, Yehoshafat was like David Aviv, who had only one heart for his Father in Shamayim.

Gra: He went in the ways of David, and the early ways of his father [Asa], before Asa sent to [Ben] Hadad.

Malbim: Virtue is gauged according to the generation. In Yehoshafat's time, one who did not serve idolatry like Malchei Yisrael, they said that he went in the ways of David. Yehoshafat went like David in the first generation (David's time); he was truly like David.

'Rashi': Achav was in Yehoshafat's days; Achav and Yisrael served Be'alim.

Malbim: Do not say that he wavered, and sought Be'alim, and only after he saw that they have no [power], he sought Hashem. Rather, he did not seek Be'alim at all.