Know that sexuality is fluid and confusing at times, and there’s nothing wrong with switching labels. This is a journey of self discovery, and there’s nothing wrong or terrible about going out with the guy if you didn’t realize that you weren’t really into guys. Honestly just explain to him that you feel like you’d be better as friends or (depending on how close you two are) explain that you realized that you don’t identify as bi anymore. I personally identified as bi for three years before realizing that I was a lesbian, and while I was at first scared of having to explain to people why I suddenly “changed my mind”, it really wasn’t anywhere near as bad as I feared, and the people I trusted as a bi girl still supported me as a lesbian. Good luck!