Take another look at why you said no to begin with. If you think you made a mistake, tell him that you changed your mind. You probably shouldn't go into too much detail with why you told him no to begin with. Likely, he'll be glad that you changed your mind. He probably won't mention your initial rejection because he didn't ask you to marry him or anything like that. It's just a date, so it's pretty harmless to begin with. If things work out, the subject will probably come up later, and you can deal with it at that time. A date is not that big of a deal, so you should really think about why it is you rejected his offer and are now reconsidering it.