Well, when he said that he didn't tell it to me, personally, we were all talking in a group when he disclosed that information (and yes, that was a very awkward moment). But, inappropriate as that was, it still turned me on, as does everything else he does all day (today it was "May I alphabetize your file cabinet for you today, Miss?" which I totally needed done, but I told him no and had another employee do it.)

As far as discouraging the behavior goes, I've tried. I've told him I'm in a committed relationship (though I didn't feel it appropriate to say what kind), and he also is in a relationship (with a vanilla who clearly is not satisfying his needs).

but the problem is that I reciprocate his service almost by accident sometimes. For example if I have to assign him a task I catch myself ending the request with "for me", two little unnecessary words that change "Please get this project done" into "Please get this project done for me." you can see where that leads.

It was suggested to me that I should tell my sub this was going on as a means of building trust and also dispelling the secrecy of it, but I'm uncertain. he doesn't have the strongest of self-esteem and I'm afraid he'll fear that is place as Mine is threatened (which it is not).