I don't see the need to tell him you are poly if you only plan to be friends. You could explain that you and your husband believe that its totally okay for you to bond and connect with other people (including men). That you are so glad this is happening with you and him.
If you are thinking that you would like something more then I think I would tell him you are poly. Then I would back right off telling him its because he has a girlfriend and you realize he might need some space to deal with the info that you like him beyond friendship. I would also tell him that when and if he is ready he can approach you to ask questions and contact you about meeting again. I would suggest you ask him to let his partner know he will be doing that if he wants to continue seeing you as even a friend. Monogamous people are usually threatened by even a friend that is interested in their partner in my experience. I think its important to respect that by respecting her and her claim to this man (even if its not your belief).
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