WAIT i have something, it’s not part of the story maybe a Pokemon saved him before the rock slide hit him off screen
and he’s hiding until it’s safe to come out agian
umm what is that ???
If he was alive, manaphy would have searched for him, but rather they decided to join Diana and eventually, you. I assume it would know whether he was alive or not due to being his partner and having the elusive power of heart swap. Also, after disbanding the infernal cult and even all cults and threats to torren such as Nyx, he still doesn’t return.
Maybe since Manaphy join Diana, it probelbly lost the power to find Calreath or Calreath knew something bad would happen to him so he asked manaphy not to find him