canine good citizen certification. At the very least, it'll be something formal to say "see, my dog is a good dog".
Second thought is just to say that he's some sort of mutt. You gotta do what you gotta do. Did you adopt him or buy him? Do you know for sure he isn't a mix at all?
My only experience is with homeowner's, not renting. Our homeowner's informed us they were not going to renew our policy when we added my pit to the insurance. We called and asked if there was anything they could do --- could we have her evaluated, etc. Evaluator came out, informed us that there was no way she was a pure bred pit, and that he was reporting to the insurance company that she was a "well behaved terrier mix".
Now, I would never call my insurance company and outright lie. But I wasn't going to argue with the guy and tell him he was wrong, and that we're sure 100% positive she's a pure bred pit b/c she was taken from a fighting dog bust We just smiled and nodded and thanked him for his time and our insurance rates stayed the same!