I'm a millennial and changed my name. I don't know why your relative wants to be called something new. There are thousands of possible reasons. You could ask them directly. If they're autistic, there's a very good chance they'll be happy to tell you their thought process in direct and concrete terms. You could also attempt to compromise by calling them "Cal" which is a more normal name. Denying them without understanding is pretty much the worst move >_>