So I'm supposed to talk to Sosiel to start his first quest, but I cannot find him anywhere. Am I just missing him in the camp, or does he not show up till later?

Most likely you are missing him.

Optional it is a loading issue. Save and reload after a proper game's restart.

I think Sosiel is bugged since latest patch, i had him with pet and I only see pet on his spot and on map, reported it in game as well, he does show later tho, so you won't miss the quest, or at least he did for me

Might have been a loading issue. I finished Lost Chapel and when i went to the camp there he was. Thanks!