asterisk8 said:

The Emperor would know about Luke Skywalker by this point. He's the guy that blew up the friggin Death Star. Word gets around.

I'm not entirely convinced of this, considering the Rebels are an underground group in hiding and trying to keep the location of their base (and presumably anything that could connect them to the Rebels - e.g. their names and involvement) a secret.

Ultimately it doesn't make a difference in the context of the line anyway. Even if he does know his name, it doesn't automatically mean he'd refer to him by it. The Emperor might be more grandiose than that. I think saying "Young Skywalker" rather than Luke suits him. Very theatrical.

Makes no difference to me either way.