Anger. Maybe a tiny bit of snapping, and more than a little despair, even if he comes back right away. He'll want to SEE and TOUCH John after, to make sure he's okay, and really there, and not permanently dead. It'll probably be a chance for John to see Eridan vulnerable in a non-calculated way, you know? In a way that Eridan isn't putting on or exaggerating for a bit more attention. It'll like, genuinely freak him out - he'll be sort of fragile like he was when he was super worried about Fef in canon, but, well...way worse. Because he saw John die.