Has he had that rash checked out and do you know what it is? I wouldn't want to fuck him if I didn't know what that was, or that I couldn't be infected, too!
I agree that your situation does not sound like polyamory. How old are you all? It sounds like there is a lack of experience in relationships, communicating, and being respectful. Seems like a lot of drama and not a great deal of maturity, I am sorry to say.
But the biggest, most glaring question that came to me while reading your story is: Why do you want to stay with him?