Hmm, see, it sounds to ME like your bf wants a second set of holes to dip his dick in. As hot as that might sound, the reality is that any kind of threesome, from one off to polyamory is insanely emotionally complicated. A few rare people seem capable of it but in my experience for most it’s a recipe for disaster.
Beyond that, there is a term for a second girl who would join a couple for sex in the BDSM world, they are known as ‘Unicorns’ because they are so fucking rare they might as well not exist.
So first, ask yourself if YOU want a girlfriend and what that would do to your relationship, and second if you do, consider if they would want him – it’s not very likely. Probably best as a fantasy to play with! At the very least, try it out a few times as a one off adventure and see if it’s something you want to do seriously.