Why was he called Pharaoh Necho?
Radak citing Targum Yonasan: He was lame in his legs.
Why did Yoshiyah go to greet him?
Radak: He went to fight him. Pharaoh Necho was going to fight Melech Ashur, who conquered Karkemish from Melech Aram. He told Yoshiyah that he does not want to fight him (Divrei ha'Yamim II, 35:20-22). Yoshiyah expounded "v'Cherev Lo Sa'avor b'Artzechem" (Vayikra 26:6 - even a sword of Shalom going to fight elsewhere), since we are doing Hashem's will. He did not know that his generation serves idolatry covertly. Yirmeyah (2:28) said "Ki Mispar Arecha Hayu Elohecha Yehudah."
Why does it say "ki'R'oso Oso"?
Radak: Pharaoh Necho saw that Yoshiyah wants to fight him, for he warned him, and Yoshiyah wore his war garments. He was punished for not heeding Pharaoh Necho's words, who told him in the name of Hashem - "v'Lo Shama El Divrei Necho [mi'Pi Elokim]" (Divrei ha'Yamim II, 35:22); Yirmeyah prophesied so (that Pharaoh Necho will fight Karkemish). Chazal say that Elokim said regarding Yoshiyah is Kodesh. Some say that "Chadal Lecha me'Elokim Asher Imi" (ibid., 35:21) is idolatry 1 . Pharaoh Necho afflicted Yehudah [from then], until the fourth year of Yehoyakim ben Yoshiyah, and then Nebuchadnetzar came. "V'Hayah ha'Yom ha'Hu la'Shem Hashem Tzevakos Yom Nekamah Lehinakem mi'Tzarav" (Yirmeyah 46:10)
It says "vEi'lokim Amar Levahaleni Chadal Lecha me'Elokim Asher Imi." Maseches Sofrim 4:23 says that both Elokim's are Kodesh. Tosefta Ta'anis (2:10) says that the first is Kodesh, and the latter is idolatry.
