I think it would depend on what your CIP Policy/Program says. Because this is a violation of CIP, you found out you don't have a valid TIN or other identifying number (if for non-US citizen), our particular program says that he would need to apply for one, or supply the actual valid number within XX amount of days or we freeze his accounts and ultimately close them if not provided within the required timeframe. I am not 100% about your loan... I am not sure under what circumstances you could call it. I just don't know enough about loans and documentation to know what provisions for that may be included in your agreements.
Another issue I see with this is that somewhere perhaps your own CIP Program/Procedures failed you. I would want to know how the relationship was established in the first place without a valid TIN. Is it an isolated incident or a potential systemic issue? I'd want to get to the bottom of that one and fast.
"I shall permit no man to narrow and degrade my soul by making me hate him." ~ Booker T. Washington