You’ll need at least three ranks of persuasion to convince Thorvald to leave his position on the island and try to escape with you (ask him if he’s a dwarf or a gnome). Then, if you want Thorvald to stay with you for the rest of the game, you’ll have to remove him from your party when you approach the entrance guards at the Wheel Clan (they’ll initiate a conversation and Thorvald will leave your party to rejoin his clan). After the conversation, it’s safe to add Thorvald back in and leave him there.
