OP....I think what you're saying is you want him to delete the number because he wants to and not because you ask him or because you want him to.
You'd like him to do it, of his own volition because he no longer wants her in his life for whatever reason.
Sometimes in life, we'll never get the answers we want even when we ask direct questions to the person.
It might be that the person doesn't want to hurt our feelings by saying why they continue doing xy or z. They might be afraid of the response if the true reason is given. They may resort to responding with "I don't know".... when they really do know.
I know you might not be able to able to see yourself in this position, but if you were the one having an affair and you knew your husband wanted the OM out of the picture, but you retained the number ...what possible reason would you have for not deleting it?
For me, there would only be one reason...because I didn't want to and wanted to keep the door open.
Because at some point, I want that feeling the OM gives me. I'm thinking that I'll loose that high I get from contact with him if I I delete the number and the thought of him totally out of my life isn't something I want.
That's the only logical explanation.
Now one could ask whether it's in my best interests to keep the number, or whether it makes my life easier to get rid of it. It might be logical to delete it, but affairs don't always follow logic.