Ok well he has interviews at other places too. This is more of a general question for how things should be done. It doesn't seem like there is a really good answer for it though.
I have been to numerous career seminars at my school and it seems like the people there always said you should bring up the two-body problem before the interview so the schools could be more accomodating. In fact, I distinctly remember numerous speakers saying it would be too late if you waited until after the interview. I didn't know there was a different protocol for applying to postdoc fellowships. The career seminars never mentioned that. I thought the speakers were referring to the protocol for academia in general.
bluejeannie, you're going to get conflicting advice on this, usually right down the prof/admin line. Administrators are going to recommend that you bring up the two-body problem sooner, because they're sick of applicants and potential hires bringing it up at the last possible minute.
Others are going to suggest that you wait as long as possible - possibly as late as when he gets an offer, because we've all heard tales of people feeling that their application was tanked because they were up front about their Problem.
My advice is, It Depends. Is the trailing position a deal-breaker for you? If it's not, don't bring it up. If it is, sooner is probably better.
There is no "general answer for how things should be done" - there are a LOT of conflicting opinions on this issue on the fora - I recommend searching through the archives. It's been addressed a lot in the past.