we have the old value, but not the new value.

Seems like in his mind, not having a new value doesn't mean it is unknown; it is the same as the old value. You can code that accordingly without explaining null because in his mind, it's never null.

Instead of arguing the point about NULLs, just make sure this doesn't create other problems. What are the ramifications of having data that are too old? When someone sees a 0% change, do they hit the panic button? Will this skew any analysis or aggregates over time?

I just don't think he needs a computer programming tutorial, but it would be nice if he knew what a null was and asked the right question instead.