His grandson Lennox, who's immortal like he is, replaced him in the same episode.
He came back though, in the episode Time for Another Ghost Hunt as a ghost, but following Ghost City's destruction, he permenantly died after sacrificing himself to The Ghost King.
The episode Weird Ways to Die revealed what caused his death was strong acid that Stephanie accidentally spilled on some explosive chemicals before the lab the team explored in exploded, which caused the acid to remove most of his skin and the explosion to severly burn him.
His death in Time for Another Ghost Hunt, was also caused by an explosion.
He stated once that ghosts can be destroyed by explosions.