I (a male Dom) yhink the fact you are discussing the question with your present slave before DOING anything speaks very well for the relationship. That old cliche about open communications being the starting point holds true.
The advice from londonswitch seems sound. I would add one or three thoughts:
--if adding a third to your menage will spoil what you have, will you give up the idea? You may think you will need to hear the sub's views before deciding that, feeling that to make up your mind before you talk marginalizes him. But-you must know yourself...if you know you are a Domme who WILL have more than one slave then you must accept yourself. If he objects, and you move ahead anyway, you will lose him, be clear about it. If you cannot lose him, don't take the new sub, make that sacrifice yourself (Dom/mes sacrifice too, sometimes, eh? "s'). Be clear with him, too: if you WILL take another over his objections, he must know that. The worst thing is to try to patch together a situation that you both know won't work, and watch it slip away.
--your threesome, if it happens, will introduce a new power dynamic. Will your present man be the lead on the chain? Some subs do not want anything to do with self-assertion;; but some enjoy helping you train the newbie under your direction....or to be trained himself under the newbie's hand, luxuriating in the additional humiliation (the newbie may be more experienced after all.)
--however you arrange it, there will be a realignment of the lines of power, and there will be jealousy.
Perhaps you enjoy resolvng that kind of continual challenge?