How do I bring it up

I'm kinda into the dd/lg bsdm thing but my boyfriend doesn't know. I want to tell him but don't know how. He's pretty much naturally dominant he won't even realize it. He also will do some things that are I guess in the sense of it a little different like pinning my wrists and flipping me onto beds and stuff and he will wrap his hands around my neck. But I don't know how to bring it up that it all really turns me on! Help!