why not be honest for once, come on, letting someone down gently - does it have to mean making some sh* up? Just tell him honestly but kindly that you're not interested in dating him. If people were more honest in life it would all be so much easier.

Even if he's hurt, thing is isn't it better that he's going to be disappointed and hurt but at least was told the truth? if you make something up then you have crap to cover up later if the subject comes up again in everyday conversation like "I don't want to date you because I'm involved" then in passing he hears you and another co worker talking bout some other guy that asked you out and the truth comes out. then he'll really be hurt. Yeah.

I figure if he seems like a decent guy but you dn't want to date him you can actually be nice and tell him no. it's not that complicated!