Is he pushing your limits or is he just sleeping around? Two very different things. If he is pushing your limits make a choice: is it something you could see exploring, or is the limit a true hard limit? Let him know. If he is just sleeping around, well... don't let him hide behind D/s to get away with it.

As Oz said, you probably won't be satisfied forcing him to do or not to do anything. That's what the "get me outa here" option is for. But I know that can be a really hard thing to do.