I don't know what to do about this situation?

So I have been in a relationship for a few months now. We had decided to keep it secret for a while. Mostly because our families don't get along very well. I ended up having to go out of town for a while and I'm still gone. I found out he's sleeping with another girl. Yesterday he told me he was gonna have a friend stay at his place for the day. I didn't think anything of it. We mostly contact each other through Snapchat. I posted on my story of something I was doing with some relatives. I noticed he had posted something on his story. Which he usually never posts anything on it. So I decided to check it out because I check everyone's really. I saw that he was literally making out with another girl. It was a video. Not just a picture. I asked one of my friends who's also friends with him. They said they had been there as well and there were signs that they had done more than just that. I just don't know what to do about it though other than break it off with him. But when I get back home I'm gonna have to see him again all the time anyways. I just don't know what I should do.
I don't know what to do about this situation?
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