Welcome to One Ring! ( /tosses coin )
Elves can be a great army but as you'll see from some posts around here they can be a little tricky to play in WotR. Their Formations are among the most expensive points in the game and I'm still not convinced that they aren't over priced a bit. That being said though, they do have some of the best Heroes, some great special rules and really are solid models. Just keep in mind that you're going to be outnumbered in almost every game so you're going to have to play a bit to really get a feel how to best use your force. I play Mirkwood Elves (sometimes with a little help from Lothlorien, Ents or Eagles ) as my main Good army (though I'm starting to build Rohan too).
For your brother, Mordor is a good start. A box of Morannon Orcs and Mordor Orcs with a couple Heroes and a Nazgul will give a nice, themed force and can give your Elves a run for their money. You don't have to be too picky painting Orcs (good for a younger brother) and they can help with the learning curve of play. Morannon Orc are tough enough to be forgiving of your mistakes and regular Orcs are cheap and expendable enough where you don't mind them getting cut down.
And for almost all Evil armies you can use these as well themed allies. Isengard is another very strong Evil army if he liked the battle in TTT.
Otherwise, he may really like Harad as an option. They can be colorful models but are still pretty easy to paint with just a few base colors. They have decent cavalry and troops (with lots of plastic model options which helps cost collecting) and fair Heroes. And of course the Mumuk which just about everyone thinks is great to see in action.
I have about 1000 points of Mordor and Isengard already and am working on Harad myself. I've only played my Mordor-based forces in WotR so far but have played against Isengard and Harad and found each equally challenging.