Co-author keeps delaying crucial part of paper I'm working on a paper with someone from a different institution. I'm first author.

The data analysis has been done, but there are several ways to interpret the result. I proposed a survey of the population under study as a way to distinguish between mechanisms and his first reaction was that he liked the idea. 

I then came up with the survey and have done all the programming. My co-author knows the population involved (as he comes from there), so I'm reliant on his connections. 

The problem is, he keeps putting it off, saying "next week, I have commitment X" (where commitment X changes according to week). 

I've talked to various peers at my current institution (not involved with my study), and all agree a survey would be a good idea. So it's unlikely that a survey is bad in the first place.

There seems to be a hidden reason for why he keeps delaying the survey release (all he has to do is to ask his contacts to distribute the survey). 

How should I approach him? I intend to tell him, basically, that I'm uncomfortable about this situation, that I have difficulty understanding the reasons behind the delay, and that if there are hidden reasons why he appears to be reluctant to roll out the survey, he should let me know. Is this is good idea, and is there anything else I can add?
I have been on both sides of that situation, so I know how frustrated you must feel.

However, the reality is that the number of "collaborative" research that fizzle out and never get finished due to lack of interest, willingness to actually do something, or time is a lot more than it should be. I could give you multiple stories from my experiences but I won't bore you with the details. Let's just say that there are over a hundred hours of my time sleeping in various people's desks and email accounts that will never come to fruition.

Your collaborator may simply be busy, or the worst case scenario is that he isn't able to actually deliver on what he promised and is stalling because they are embarrassed and/or don't want to admit to it (I've had this happen). Regardless of the reason, this is not a good situation, for both you and your collaborator.

Face-to-face and/or heart-to-heart conversations have resolved such situations (in one way or another) for me, but I can't say that it works for everyone. Still, it wouldn't hurt to try. Just let them understand you simply want to know whether the project is still alive or it's time for you to either find other ways to get the project finished or simply move on.