“Just be sex”… That truly is all it is, but with fun added in. It just pleasing sex. The more y’all read and discuss consensual non monogamy, the better he will be able to participate. The only emotions that should be present is “happiness” for both of you.
We never included a third in our bedroom. She just wasn't up for that. Did have a dear friend live with us for several years. We became a poly family but again no threesome. We enjoyed ourselves, always just two at once, many times until our friend moved due to work requirements. Then on occasion I would visit by myself which led to us sleeping together as did my spouse. When we visited together there was no sex. Not one time did my spouse want us all together at once. She just wasn't emotionally up for it and no discussion or reassurance was going to change her mind. My point is it must be left up to the BF to decide if another person being added to the relationship, even temporary, is right for him. The suggestion of attending a sex club is a good one. BF may be up for that and it can be a good test of his resolve. The realization that a different penis is sliding into the owner of the pussy you deeply love can lead to a deep emotional trigger. Very few people can handle that.
I guess you must like sex to want 2 men. Tell him what you want, maybe bring it up as pillow talk. Watch a sexy movie together.