I don't think he is melting.
First, the pressure and heat needed to actually melt bones is insane. He would burn and turn to ash before melting.
Second, if you check the panels before his death, the leather of his gloves and boots is also "melting"...
That's why I think he's actually getting liquefied. By magic or chemistry, it actually looks like something changed him and his gear.
You can clearly see his ennemies turning to dust, so if I have to guess, the powder he poured on himself maybe does something strange under high temperatures.