Guy is telling people we had sex even tho we didn’t!

Okay, so I’m at my moms hometown where she grew up and everything for summer. The town had annual party’s, it’s a big thing you know a lot of people from nearby towns come and you know it gets fun. Well anyways I was just talking to this guy outside my grandmas house you know he was just asking me about where I was from and things like that well today my cousin came to me and told me that the guy is telling the whole school that me and him had sex and a bunch of stuff even tho, it’s not true! And you know I’m mad and I’m not sure what to do. :( any advice?