I think if it were me I would either wait for a moment when I feel confident that he is to be trusted or just do it. I have said, in the past, to new co-workers, "so, just so you know, I identify as polyamorous and here is what that means to me. I wanted you to know just incase you see me or my husband out with other people and think we are cheating. If you ever want to ask questions or talk about it with me, I would be open to that." Then I've let some silence occur incase they want to say something and if a conversation doesn't start then I move on to another subject. Usually they thank me for telling them and say its interesting and relate a personal story and that's it. I think approaching it in terms of a co-worker first might be your best bet so as to not freak him out about your feelings for him. Later, if he shows interest and you've had enough evidence that he is interested in you, you could bring up the possibility of more with him.
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