Show him a 7911 phone - it doesn't have physical buttons that can be be used for the purpose.
Have ready some document explaining that softkey cannot be changed from pre-defined purposes.
Mention that they will need to buy a bigger phone if they really wast single-button calling - you will notice the urgency for that to disappear instantly.
Note - all the above is moot if you're willing to use, for example, "6".