I am co-owner of a 2-family rental property, and the other co-owner now lives in one of the units, so he contributes to the mortgage but doesn't pay rent. How do I report the rental property correctly?

I can't report this as a "rental property where you live" because I don't live there, but the other owner does, so it is owner occupied, and he doesn't pay rent.  I can't find a place to report that.  I'm afraid if I report the whole property as a rental, even though he doesn't pay rent, I will be claiming an undue loss and getting an undue tax refund.  Can you help?