You guys are too focused on levels, so you cannot see anything beyond. :-/
Once Halsin would not have any number visible next to his name, nobody would say half word about how he "never can be Archdruid". -_-
So ... we all can see that levels are nice way to express potential power of character, but we also SHOULD be able to see that in this particular case it would be extremely limiting from story perspective and will cause more problems than how many benefits it would bring (yeah you guessed right, none).
So lets ask Larian to rename his title to semi-Archdruid, Archdruid-Junior, or Acting-Archdruid or whatever else shit you can made, since mine vocabulary ends here. :-/
I think he's just with stats like this until we have more content. I doubt he'd supposed to be such a low lvl. He and maybe some other non origin companions (if that happens) are meant to enter the group later, or replace companions we may lose.
Exactly what i say for around half year around here ...

I believed that we will resolve our tadpole problem at Moonrise Towers, maybe not completely ... but at least to some point where our characters will believe they do not need to worry about turning into squid any time soon.
Then, as Larian litteraly said to us, we would have to "commit" ... wich i personaly (on the contrary to majority of this forum, who for some to me unknown reason take it as "every companion you dont have in party in that moment will die") understands as "you will pick one Origin character, and follow his personal quest ... maybe others who will like you will join you, unless their personal quest will lead somewhere else".
And THAT in my humble opinion would be perfect spot for Non-Tadpoled (read as Non-Origin) companions will be presented to us. :P
And yes ... that means Halsin, that probably could mean Minthara, that probably could mean Alfira (once we reach Baldurs gate, or meet her in our way there) ... etc. etc.
But it seems i was kinda wrong. :-/
Since in last Panel From Hells Swen litteraly tells us that they are "concidering to allow level 5 in Act 1" ... wich could (and i would like to point out that this is just mine interpretation) mean that level 4 would not be just cap for EA, but for whole Act 1 in full release (is it just me, or does that really sound like worse idea ever?). :-/
Wich would mean that since Halsin is actualy level 7 (unless i remember it icorrectly) ... we would probably reach his power level (and therefore he would be aviable for us as a companion) somewhere around end of Act 2. :-///Edit: (after watching several videos on youtube)
I find out that Halsin is actualy level 5 (so yes, i remembered incorrectly) ...
So, act like i didnt say anything.

He could follow our group without any power-struggles.
who calls him an archdruid? I mean, I believe I talked with everyone in the grove and everyone just talks as if he's a druid, no one ever mentioned him as an Archdruid from what I can remember. This is an honest question, I'd like to know which npc says he's an Archdruid, I'm always looking for new dialogues, etc that I can find.
Nope ...
The title he uses when he introduces himself to you is:
"Halsin, First Druid of The Emerald Grove"
