My boyfriend wants to swing?

Hi, me and my boyfriend have been together for a year and a half. We’ve been through a lot of ups and downs mainly because of his self-issues because I used to have multiple sexual affairs in the past. He was a virgin when he met me and this affected our relationship so much that I also lost my self-worth and value. However, he grew up and got over it. We love each other so much and we plan on getting married soon.

Right now, he wants to explore things with me. He told me he wants to have a threesome with a friend or somebody we know. It bothers me because we used to have an issue about this kind of thing, and now he wants to explore. I think it’s a trap that would just lead the both of us into much much bigger fights, but I can also see he’s willing to do it. We’re young, (He’s 21, I’m 20) and I want him to experience new things because I’m afraid he’ll get bored of me soon, but I’m also scared that he might like it so much he would cheat on me with the girl we want to sleep with. I’d like to explore with him, too but I’m just so scared? I’ve been thinking a lot of “what ifs” although I’m willing to do it, too.

Do you have any tips about swinging and exploring new things with your partner? Would you please give me any tips? How can I trust my boyfriend that he is not gonna cheat on me after this? I’m new to these things.