How do I tell him

I’m in a bit of a sticky situation girlys and I need your help..

So we’ve been together 4 months now (we’ve known each other nearly 7 years and been seeing each other for 6 months). I live in shared accommodation and I’ve slept with someone that lives here.. it was years ago now and we don’t speak so it doesn’t bother me but they play pool together and they get on well.. I just don’t know how to tell him and I don’t want him to feel like everybody’s been laughing at him