Pardon my intrustion into sub space

I would suggest you find a reasonably short but quality piece describing what bdsm is (specifically, is to you) and send it to him with a small intro, along the lines of "This seems kind of sexy. Does this sound like our relationship to you? Would you want us to go towards this direction?" That shows your interest without promising or demanding anything or instigating massive changes (all of which freak out partners), and invites his response and a real conversation. I suspect a few "I like this, but not this..." lines will help both of you along. If he's already acting like a Dom, then although he may not want the title/want to be part of a "lifestyle", he may be a lot more open to the concepts behind it (which is the important bit anyways, yeah?)

Good luck, and keep us posted!