Who is Shalum?
Rashi (from Horiyos 12a): He is Tzidkiyah. It says "ha'Revi'i Shalum" (Divrei ha'Yamim I, 3:15) because he was the fourth son of Yoshiyah to reign (he was the third son born to Yoshiyah). Yeho'achaz, Yehoyakim and Yehoyachin (ben Yehoyakim) reigned before him.
Radak: He is Yehoyachin (ben Yehoyakim). He is called Ben Yoshiyah, for grandsons are like sons.
Ibn Ezra: He is Yeho'achaz. He died in Egypt. Radak - I explained this in Sefer Melachim and Divrei ha'Yamim (refer to Melachim II, 23:30:5:3*).
Malbim: He is the fourth son of Yoshiyah (Divrei ha'Yamim I, 3:15). Even though he never reigned, it says "ha'Molech" for he was proper to reign 1 , and this is why he left (Nebuchadnetzar exiled him, lest he contest Tzidkiyah's kingship).
Why should he reign before Tzidkiyah? Perhaps Malbim holds that he was born before Tzidkiyah; Tzidkiyah is listed before him, for Yoshiyah's sons are listed in order of when they became king. (PF)
Did Shalum leave now?
Rashi: No. He prophesizes about him, as if he already left.
Malbim: Yes.
