I actually don't trust him. This game makes pretty darn sure that you don't easily trust people. A lot of people have hidden agendas. Too many, in fact.
If you talk to the adventurer idiots later, the leader says Halsin was gung-ho about joining them when he learns about the Nightsong under the temple and seemed more interested in THAT. He then later wants to join you when you go close to that relative location too and puts someone nameless in charge in his stead. He also kept putting off talking about your tadpole just to tell you to go somewhere else after all that you do for him and the grove.