Interesting. This is the opposite problem I have with my own wife.
For what it’s worth here’s what I’d do. I’d watch some porn with your boyfriend that had a MFM or MMFM scene or two in it. When one of those scenes are on - perhaps when the starlet is getting a cock in her mouth and pussy at the same time - you moan a bit and say ‘that sure looks like fun’ and wait for his reaction.
I think it really also depends on how much you and your boyfriend have done so far and shared with each other. Do you use toys? Does he ever finger your ass while he fucks you? Any of those things could start you down the road to talk about those fantasies.
Or last but not least, ask him what his biggest sexual fantasy is and if he’s a decent chap he’ll ask you what yours is in return.
Let us know how it goes and I can either make it public or keep it private. Good luck! We need more women like you in the world.