The demo shows that he will probably join us after Alexius' defeat.
In the part of the demo that we haven't seen yet, but that people have summarised on the internet, I'm pretty sure either Mike or Mark said that they were switching Dorian into the party because they wanted the extra content with Alexius. That would imply he joins before then.
The Inquisitor also has a chance to give Dorian her condolences over them killing Felix and/or Alexius, which implies that Dorian was around to tel that story of his connection to those characters some time beforehand.
I think the Redcliffe confrontation with Alexius was a fair way into the game, and it's the second major quest there - as in, we solved some kind of problem there and then later Alexius moved in. The same thing was said about Crestwood in the PAX 2013 demo - we were returning to that location for a second time after already dealing with something there earlier.
So long story short, I think we recruit Dorian much earlier than the second Redcliffe quest. No idea where, though.