you don't have to say you're poly necessarily, if it isn't comfortable. Especially if this is not going to be a romantic relationship... if it were, that would be different. You can just say you are dating already and that your life is fine the way it is... you are just looking for friends right now. He probably wouldn't suspect then and if he has his ideas? Who cares? It's not his business anyways. It might be wise to talk to your couple about what they would approve of if he or anyone else asks direct questions. THAT is a discussion worth having and thinking about, because it could very well come up!
Anyone want to be friends on Facebook?
Send me your name via PM