Threesome advice

So my boyfriend and I are kind of on the rocks right now, been together for almost two years but at the moment we decided to take the title off because shit was hitting the fan. Anyways, we are very sexual people and he really wants to do a threesome, and will even allow me to pick the person. As much as I want to be down for it, I don't think our relationship is secure enough to bring in a third person. I try to explain that to him but i dont think he gets it. I dont know how else to get it through his head. Anytime he brings it up it just bothers me because i feel like maybe he just wants to hook up with other girls but he tells me he wants to see me hook up with them.