Why did he mention the bones? Surely the flesh had not decayed yet! Also the elder, presumably, his flesh will be intact when they bury him!
Radak: The bones are the foundation of the body.
Ralbag: He knew that Yoshiyah will not burn the bones of Ish ha'Elokim; he commanded to be buried next to him, to save his own bones 1 (Melachim II, 23:18).
Malbim: Their custom was to gather the bones after [the time in which we expect] the flesh to decay, and bury a second time. He commanded that both the burial and gathering of the bones be next to the Navi.
Radak (23:18): There were buried in the same grave. Metzudas David (23:18) - the elder was buried next to Ish ha'Elokim. Yoshiyah feared to take the elder's bones, lest he move the bones of Ish ha'Elokim.
