Well, you couldn't ask him? I know that is kind of...blunt. I suppose it would depend on how long you two have been together and what level the relationship is on. Honestly though, if it wouldn't take the "fun" out of it, I would just ask him...at least somewhere down the line. It's not forcing him, it's basically just letting him know what you are into.

I'm not really sure that this "advice" helps, but in that situation, if the relationship was to the point that I thought I could tell my partner pretty much anything, I would let him know. Perhaps he's just not sure of exactly how far you want to go?