I was in a similar situation and it has, thus far, worked out fine for me (knock on wood). I would travel back and forth from US to CA to visit hubby prior to us being married. Once, after being married, I was briefly "held up" and told to go inside to see Immigration. I had no clue why. They issued a visitor's record to me after finding out that we were still working on the PR app forms. They said it would be helpful to have this as I traveled back & forth quite a bit. Anyway, that visitor's record expired and on another trip back & forth, I stopped at Immigration to see if I could be issued a new visitor's record. I brought the pr app with me to show them the progress that had been made. They issued another vr to me good for one year as they said it could very well be longer than that before I heard something. Now, this vr doesn't GUARANTEE re-entry for me, but I have not yet had a problem. (Again, knock on wood.) I have my visitor's record, I bring along copies of necessary paperwork to show that I have a file #, etc and app is in process. I always tell the truth when I'm coming & going. I really have never been given a "hard time". Perhaps, I'm lucky, I don't know.
I would suggest looking into the visitor's record. I'm sure having this document might help. Again, it doesn't guarantee re-entry. If he does travel back & forth, I would bring along proof of his ties to the US, still.
Oh, and we, too applied outland. Hope this helps ease your mind a bit.