You can tell him that you're poly in the context of just telling him about who you are, and the way your life is. It doesn't have to be presented as "I'm poly, and I like you, and if you like me, we have some options to discuss..."
You can also tell him what poly means to you, and your relationship ethics, and the difference between poly and cheating.
And above all, if you think you're starting to feel a little hot and bothered when you're around him, but he is in a monogamous situation and it would violate your sense of ethics to facilitate him in cheating, then try hanging out in public spaces, or with your husband there, or maybe less often. Avoid temptation. If he is as young as you are, and his LDR is that new, it might not be a permanent situation-- but you don't want to be the catalyst for it ending.