Hmm, definitely a lot to think about - but I am pretty sure that as long as he's in the relationship, I am content being friends, and as of now, I see no reason to tell him I am poly or see the potential to be more. I actually look forward to a long friendship between us and if he magically is ever single again, I might tell him at that point. I definitely want to be careful before becoming involved with somebody on such a romantic level again.
I don't tell very many people about my life style choice because it's private in my eyes and this guy is roommates with some good friends. I am thinking it's not necessary to tell him at this time and possibly make things awkward, especially since nothing can really happen anyways. Plus I think he already gathers that I am in some type of open situation, maybe just not the specifics.
I really appreciate the different approaches and perspectives. Thanks for the replies! I'll keep this situation updated.