Your best bet at determining where things stand in that regard would likely be to invite him over for a night out, or for that matter, invite him to a public place where you can be outside the work environment and get to know each other. Often times in general conversation outside a workplace, you can get to know a great deal about a person, including whether or not they're into you.
If possible, try to subtly steer the conversation towards him to try and get him to reveal key information that may give you an indication of whether or not he's even gay. The last thing you'd want to do is make a move only to find out in a very unpleasant way that he's straight as an arrow, or worse off, married with kids. If all goes well, you should be able to eventually get to the point of being able to pop the question directly.
Chances are that if he's making such excuses before work begins to be able to make time to chat with you, he's at the very least interested in a friendship, but more likely he may have a hidden agenda wink wink that he may not necessarily reveal right away for fear of being slammed with a sexual harassment case. If this is the case, he'll be likely to jump at the opportunity to arrange something outside of work.
