VADER: What is thy bidding, my master?
EMPEROR: There is a great disturbance in the Force.
VADER: I have felt it.
EMPEROR: We have a new enemy...the young rebel, Skywalker.
VADER: Yes, my master.
EMPEROR: He could destroy us.
VADER: He's just a boy. Obi-Wan can no longer help him.
EMPEROR: The Force is strong with him. The son of Skywalker must not become a Jedi.
VADER: If he could be turned, he would become a powerful ally.
EMPEROR: Yes...yes, he would be a great asset. Can it be done?
VADER: He will join us or die, my master.
Having giving a listen to all the various versions, I reckon there could be enough spiteful 'inflection' in the words "The young rebel" and "Skywalker" that is spoken in the SE dialogue that could be combined to make a good-sounding 'beat' that matches and flows well with the rest of the "We have a new enemy..." line.
Like others, I thought about removing either "...Luke Skywalker" or "...young Skywalker" from the sentance altogether too, as the 'beat' of the dialogue still sounds good without it, before Vader eventually replies "Yes, my master". But I ended up thinking that without clarification of exactly who the Emperor means before Vader responds "He's just a boy...Obi-wan can no longer help him", that the Emperor might reply "Not that brat Luke you fool!, I'm on about ANOTHER enemy we've got to contend with!"
The problem is that because the movie has shown Vader actively searching for Luke by the time this late scene occurs, the Emperor's message gives the impression that he's only NOW discovered that this Luke Skywalker fellah could be a serious problem, and didn't know that Vader was already on his own agenda looking for him while chasing the 'Rebel Alliance' with the Emperor's fleet since the start of the movie.... So I don't think Vader would jump the gun and give away the fact that he knows exactly who the Emperor means by saying "He's just a boy...Obi-wan can no longer help him", WITHOUT the Emperor mentioning his name first. Although it could be argued that this scene was just oddly structured late into the movie in the first place, rather than early on, I prefer the scenario where the whole scene can be seen has having various possible 'suspicion'/'deception' connotations between the two characters at this point, since the Emperor MAY have only decided to let Vader know at this particular point that he's aware about Luke's potential...but ACTUALLY knew much sooner, and decided not to emphasise it with Vader until this point...for devious reasons...
It's hard to say just HOW much they discussed about Luke before this point, and not that obvious that Vader should be seen to know exactly who the Emperor is on about unless he tells him. If they are already on the same page about it all, then it seems that "We have a new enemy..." would be an strangely unneccessary thing for the Emperor to say to him this late into the game. On the other hand, by bringing it up at this late stage and saying something like "...the young rebel, Skywalker" at this point in the proceedings could imply that the Emperor (and Vader?) has indeed ONLY JUST become aware that this particular rebel (who just happened to be the guy who managed to blow up their Death Star) was 'turning' (Force-wise) into a bigger danger than originally thought, and that they were on the same page from earlier in the movie as far as concentrating on their 'main' enemy - 'The Rebel Alliance' in general, with this Luke guy as just an extra 'bonus' amongst them, to begin with...
My head hurts!...
Anyway, I really dislike the new retconned SE dialogue changes, so even if my own slight alteration above doesn't come about, I'm more than happy to hear either "...Luke Skywalker" or "young Skywalker " instead, as long as the Emperor still says who he means to Vader first.