I am assuming your relationship is a long distance one? That he is telling you is sort of a good thing, it means his line of communication is open with you. The other side of that coin is it doesn't seem you are okay with it.

Have you and he worked out the issue of him being poly? If not, I suggest you address it now, rather than later. Just because you are collared online, doesn't removed your right to make choices about the pending move. Should you not be okay with his terms of what your life will be like when you arrive.

Perhaps negotiating a contract between the two of you would be an idea to consider. There are many out there, for all kinds of relationship types.