If only he is gone, you can just leave him for dead, finish the game, then save him in the next playthrough and see what changes with him around. That way you'll get to experience both versions.
Depending on what you did and didn't do before moving into the Shadow-Cursed Lands (and the game does warn you that quests will be irreversibly progressed if you do), I'd be more worried that you lost a lot more relevant NPCs than just Halsin. And a bunch of important and interesting cutscenes and dialogue with companions that you might not want to miss out on in your first playthrough because it adds a lot of character development.
