Halsin should be at least level 10 IF they make him a true member of the party, which, storywise, they should. Makes no sense for him to say he'll come with you but he'll just sit at your camp and do nothing. They have to at least come up with a REALLY good reason why he doesn't join you in fights.
So, the issue with him being a party member is he SHOULD be OP compared to the rest of your party, which then throws off balance.
So, good luck, Larian, solving that issue you created for yourself. Storywise, he should be a playable party member, but gameplay wise he should not for sake of balance. I personally would like him to be a playable companion, but only if done right.
It's too late to make him a companion since he's already been given such high status. What would make sense is Halsin sending a novice like Nettie with you as a companion. But the internet isn't thirsty for Nettie. They want an alpha male but Larian is only giving them intellectual, flamboyant and magical.
Maybe they are planning for Halsin to have a Gorion moment at Moonrise instead of making him a Druid merchant for camp or whatever. That would be better storytelling already.
Larian doesn't seem to think powerful NPC's sitting in your camp with nothing better to do is a problem though. We already have an undead there with unlimited magic power for player convenience. They always sacrifice storytelling for gameplay mechanics. Even when it's not necessary, like mr. skeleton.
But seriously, Halsin should stay at his circle to
lead. They are just recovering from a hostile Shadow Druid takeover. What kind of a leader just takes off again and leaves his flock to their fate?
