I'm going to answer myself here and say it's up to the DM.
This because it looks like they are indeed treated as 2 different entities in the Sword Coast Adventure Guide, but I guess that a DM can keep the 4e twist if he really liked the fact that they were the same deity with only different aspects.
Things might change when we have more books published, but this is the present situation as far as canon is concerned.
UPDATE: confirmed they are different entities again in 5e -> https://twitter.com/Sernett/status/877036497165139968
There is no right answer to this question, but there is a guideline:
I don't have the DMG/PHB right now to point you the exact quote but in short: The DM decides what to roll.
There are conditions that might favor what you're trying to do (intimidating thugs after you just rolled a critical and killed their boss), and there are conditions that makes things harder (trying to talk your way out of a murder acusation after being found with the murderer's weapon).
Note that none of this situations are defined in the rules, it all comes down to the DM's adjudicating the situation.
There are rules for combat that tells you what to roll and when to roll, there are guidelines to challenges but ultimately, it's all up to the DM, so everything I say after this is utter bullshit if your DM says otherwise but;
Option 2 sounds quite about right, but, if I were in your DM's shoes, I would just give the elf the answer right away. Yes, you DM might rule that this particular task requires no roll due to the conditions.
It makes complete sense that he should know this alredy, hell, in fact it hardly makes sense that he doesn't, if a player invested on some sor of background, he should have that reflected on his knowledge about the world and how he interacts with it.
Best Answer
The best resource for D&D elves, in my opinion, isn't any book at all, but rather afroakuma’s So You Want to Play an Elf. He is probably the foremost expert on D&D lore on the internet (at least, of those who don’t work for Wizards of the Coast), and his work incorporates decades’ worth of lore from the first, second, and third editions of D&D.
To be sure, his article on elves incorporates his own ideas not mentioned—at least explicitly, in these terms—in any D&D book. But they are extensions of the existing lore, and pains have been taken to make sure it is all official-compatible.
And, as mentioned, it doesn’t include material from fourth or fifth edition. Again, things are more-or-less compatible, and earlier editions had far more material than either of those do at this point, but some care might be necessary on some details if, say, playing with a 5e purist.
But the details are, honestly, the least important part, to me. And if your son has just started playing, there is a good chance he’s not a purist. So I think this article can be immensely inspiring for you.
In particular, the concept of short time and long time is the best idea I have ever seen for explaining elf mentality, and how their longevity affects them. It asks an elf to be more than just a snooty, tree-hugging human, but instead actually different. So that is the part I want to most highlight, though all of it is great.
This long-time is a deeper explanation for elves’ “harmony with nature” and how they can spend so long doing things that other races do more quickly, efficiently, and brutishly—elves naturally perfect their actions, not to maximize their effectiveness, but for grace and harmony.
It’s also why elves can seem so annoyed by other races—long-time tends to be impossible around those who cannot experience it.
The other crucial thing about elves that afro emphasizes—and this is more directly from D&D lore—is the fall of Araushnee/Lolth. That Lolth is a fallen elf goddess is well-known, but what is often forgotten is that she was the elf goddess of passion and destiny.
This too goes a long way to explaining elves, their almost passive contentment, and their ceaseless hatred for Lolth and her drow.