I would simply tell them, "I don't care. That is not his legal name and listing him as such is improper."
Names do not relate a person to anybody necessarily. If it did, there would be a huge family reunion for the Jones' every year. There are other methods of notating relationships. Names are actually a very poor method, especially since our country is so well known for divorces, multiple marriages, non-married mothers, and the such.
I can see where this would even lead to problems. In the future, if you list him by his proper name and records are searched for the correct name, the incorrect name may not be returned in a search.

