Rashi: He was the youth of Elisha ha'Navi. Targum Yonasan says that he was his Talmid. Radak - he holds that this is Samuch, it is as if said Na'ar ha'Navi.

Malbim: This teaches that the Na'ar himself was a Navi, and added matters that he knew via Nevu'ah; he knew of Eliyahu's Nevu'ah. Elisha commanded him only to anoint Yehu king over Yisrael.