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.
The most crucial element to understanding an elf's mindset, though, is to understand long-time and short-time. The elf brain does not perceive time and receive information in the same fashion that most other humanoids do; elves live much of their adult life in a pseudo-trance known as long-time, in which they can idle away days or weeks at a time without devoting significant mental focus to anything. As creatures with a lifespan measured in centuries, elves have a lot of time available to waste on getting things right, and long-time is the biological mechanism that allows them to do it.
An elf is in long-time whenever there is nothing stressful that requires true focus. Elves in long-time can eat, drink, read, play music, craft woodwork or even hunt game or fight goblins if they are sufficiently at ease with the task at hand. A sort of mental auto-pilot, long-time supplements the elven need to trance by allowing elves to keep their minds at ease while developing muscle memory and picking up on subtle nuances and details that stand out as relevant. Elves in long-time can devote themselves to a given pursuit for anywhere from a week to several years without even having to notice what they are doing. Long-time is a fluid, waking dream; peaceful, functional and allowing the acquisition of experience over a long period of time. Elves can do many things while in long-time, but unless stressed, they will not be devoting the necessary focus to noticing much of what they're about.
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.
Now that she has reclaimed her divinity, Lolth still controls a twisted version of her old portfolio but rarely exercises it. The loss of Araushnee's portfolio has been a direct contributor to the stagnation and slow decline of the elven people, and one that Corellon cannot rectify. The only hope for the elves lies in finding those whose essence was not scoured - the few who were taken by Gruumsh when he stole away the Misty Isle.
This too goes a long way to explaining elves, their almost passive contentment, and their ceaseless hatred for Lolth and her drow.
