He's as legit dead as anyone in that world, but unless the actor didn't wanna be on the show anymore.. I'm thinking that'll be part of the season's arc, getting him back somehow. Maybe Julia does become a Goddess again, or Alice as head of the Library, and figures out where the underworld metro actually goes. Possibly Our Lady Underground actually died to the monster, and Julia takes over some of her responsibilities, which include power over the underworld.
Actually, fuck.. just as I was writing this, I decided to do some googling, and it appears the actor actually is leaving the show, according to the producers. So.... he really is dead. Damn, that sucks. It made for a great moment, but it doesn't feel right. They got him back together with Alice just to make the finale more gut wrenching? That's so shitty. Really, the finale was kind of weak except for that moment and the ending scene. Granted, they were REALLY good scenes, but it feels like they couldn't come up with a satisfactory resolution, so they decided to just do something drastic. It does explain why they moved towards making everyone else the 'main' characters this season, but still..
Oh, and considering it was brought up recently in this thread, guess we are back to making time move differently in fillory?