I see a couple possibilities. One, he really likes you a lot but refuses to move forward for fear of screwing up the pleasantly casual relationship you currently have (which, he figures, having seen too many John Cusak movies, may one day lead to a drunken after-work hook-up which will then blossom into love), which may not be much but at least allows him to see you every couple days.
Two, he is just a friendly guy who flirts without meaning anything by it and now is at sixes and sevens trying to figure out a way to let you down without embarassing you.
Third, he’s attracted to you but for whatever reason (including possibly a girlfriend) he intellectually feels your romance would never work out. As long as you don’t go on a date, he can convince himself that you’re just pals and, like situation #1, he can keep hanging out with you. If you do go out on a date, it’s no longer casual banter between coworkers, and whatever (currently only incipient) rationales he may have for not getting involved with you crystallize.
If you’re willing to risk a bit of heartbreak (in a way I never was when I was single), here’s what you do. Next time you’re about to see him, be looking over movie times. Have a specific film in mind and tell him you’ve been thinking about seeing it this or next weekend and would he like to join you? (I recommend Pirates of Caribbean or School of Rock – both fun and unchallenging, but much better than most films which fit that description. Although with Pirates you run a big risk of him having seen it already.) If he says he’s seen it or he’s got plans but seems conflicted over the missed opportunity, suggest something else, but have it be a specific thing on a specific date. (But again, it should be something you were “planning” to do anyway – maybe a museum.) If he refuses the film without making the excuse, or if he rejects the back-up offer, then he’s in either situation #2 or #3 and it’s never going to happen for you. Maybe someday he’ll actually ask you out directly, and maybe you can still keep chatting as you do, but you should resign yourself that there’s no real chance of a romance developing.
If he is willing to go to the movie or the back-up with you, kiss him on the first date (if you’re still interested at that point, of course). His acceptance of the date means he was in situation #1. That means he’s squirrely. If you kiss him, it’s a very clear signal that you’re interested, which may be what he needs. Trust me, I know about squirrely men – throw whatever signals you want; if it’s not physical we can always find some other explanation. (Actual thoughts that went through my head during my single days: “Maybe she just likes the way the denim stretches over my knee.” “Maybe she just really enjoys tickling people.” “Maybe she likes telling guys she doesn’t know all that well that she’s not wearing a bra.” “Gee, I guess that joke was really funny.” “Maybe she just really likes the new underwear she bought and thinks I’d appreciate the design.”) See, it’s not that I wanted any of these to be true, just that since the possibility existed, there was some chance I’d be rejected if I tried anything, and it’s better to be alone than to be alone and laughed at.