If he’s bored already, he’s not gonna level up to 120.
To answer your question:
- Silverpine Forest, absolutely recommended for lore on the Undead plus a great zone to level in.
- Stonetalon Mountains has some rich lore on Garrosh’ expansion lust. Same goes for Ashenvale, which also provides some Night Elf lore.
- Azshara for Goblin stuff.
To be honest, leveling to 120 is mostly about killing boars and gathering herbs. It’s not going to get any better if he is bored already. The best you can do is read up on the lore yourself, and lead your friend to the zones while telling him about the context.
So for example, you go to Silverpine Forest, and you can tell him about the Scourge, and about Sylvanas. You’ll also cross Gilneas, and you can tell him something about the history of the kingdom, and the invasion of it by the Horde.
When you go to Stonetalon, you can tell him about how Garrosh wished to expand further into Kalimdor, how he believed that the Horde should’ve conquered Kalimdor, as his father probably wanted.
And make sure you do the Dungeon quests. When you got Dire Maul, you can tell him about the Highborne, or when you go to Uldaman, you can tell him about the Titans. And it can’t hurt to take a break from leveling sometimes.
If he is here for the lore, try to be his tour guide. Just giving some context about as much as you can might help a lot, and make the world feel more alive.
Feralas? Highborne, Green Dragons, Emerald Dream, etc.
Dustwallow Marsh? Theramore, its destruction, the invasion of the Barrens (building the Highway), the Black Dragons, Onyxia, Varian, etc.
Felwood? Something about the War of the Ancients, Skull of Gul’dan and Illidan, his confrontation with Arthas, etc.
Lore is everywhere. You just need to know where to look and what to tell.
So in terms of real leveling paths, I can’t help you a lot. Most questlines are basically sidetracks, leaning on the established lore but not giving a lot of new information, apart from a few exceptions. But if you know something about the lore yourself, you should be able to give him all the information he needs to know.
If you have any questions or want some specific info, let us know.