You could start with subtle things you say or do and see how he responds to them. That way you can gauge his reaction before actually having a conversation about things.

Although I'm not sure I'd have a big "let's sit down" talk about it, because it could come off as intimidating to him, or he might think it's a criticism, and the conversation might turn into an argument. Whereas, in the throws of passion, you could suggest he tie you up, or tell him it's ok if he smacks your ass.

Do you already use dirty talk at all? If you do, it'd be easy to ask different questions or suggest things that would bring out his dominant side. If you don't, you could start and see how well received that is with him. Before I ever got involved with a Dom, whenever I would call a guy I was dating "sir" it would almost universally be well received and bring out a bit of their bossy side. You'll find out pretty quickly just by making suggestions, experimenting, and seeing how that goes. The conversation might happen afterward, "So, did you like that? Would you want to do that again?" etc. With one of my exes, I had asked him to hold me down and/or use some sort of bondage and I figured out pretty quickly that he wasn't into that, while another ex just tried it one day and was surprised by how well I responded to it.