A guy friend keeps asking me when we are going to go out, again? HELP!!?

We went out about 5 months ago...platonic...for drinks. It didn't get romantic, but I had a feeling he was possibly trying to take it that way. We had never been out like that together, before. Long story short, it was late, I wasn't drunk but really tired from the wine, plus I live a long way, so I slept on his couch. (Nothing happened. Like I said, still platonic.) Now, every time I see him, he mentions, "We should go out again like that!" or "When are we going to do that, again?" He says it EVERY TIME I talk to him, like once every week or two. So why hasn't he officially asked me to go? I always say, 'Yeah, we should. Just let me know when and where." And he never follows up. Why does he continue to bring it up...I never do...and then not do anything about it? The first time we went, I am the one who made the 'date.' So what gives? Why doesn't he make a date me?? And WHY does he keep bringing it up??