Yikes...

So I was REALLY desperate to get into a relationship, and I was talking to this boy that I used to like but don't anymore. We were both talking about how we were single and I asked him out as a joke. Before I could say that I was kidding, he said yes. I couldn't tell him afterwards but I honestly don't want to date him because I have a crush on someone else. I wanna let him down easy but I don't know what to say because he is already posting about me on Instagram and we got together yesterday. HELP