Dunno what budget you have, but check out The ABCs of D&D, The 123s of D&D, and Monsters & Creatures: A Young Adventurer's Field Guide.

Depending on how much he already reads, the ABCs and 123s might be a bit too childish for him. They're fairly typical alphabet and number books, just very well done and with good art. "A is for adventure, where we have lots of fun. B is for beholder, that sees everyone" or something. I kind of adore the books and my niece (now 4) loves them.

We just spent some time with the niece recently and I gave her the monsters and creatures book. She didn't really understand what it was, but when I told her it was a lot of the same things as in her ABCs book, she was excited. It has ~5-6 chapters, each about a different biome. The first is the underdark, the second is the forest, and I don't remember the rest. For each biome, it tells you about different creatures that live there. In the underdark, you learn about the beholder, bugbears, carrion crawler, and I think a few others. It has a good picture of each monster, a bunch of information about its various abilities (and even goes over every separate eye beam for the beholder), a picture that compares its size to a human, and some tips for fighting them.

Monsters & Creatures is part of the Dungeons & Dragons Young Adventurer's Guides series. Warriors & Weapons and Wizards & Spells mostly consist of the following:

1. Breakdowns/overviews of each class (martial in Warriors and magical in Wizards)
2. Information about famous members of each class. Can't remember any in particular, but maybe Mordenkainen for wizard or whatever.
3. Class selection tree for the classes in that book - "Are you fast?" select yes "Are you sneaky?" select yes "You should play a rogue" etc.
4. Information about weapons or schools of magic (and spells in each school)
5. Something else. Warriors has information about each race. Can't remember what else is in Wizards.

Overall, I don't particularly like those two. They're just not as good.

However, Beasts & Behemoths and Dungeons & Tombs have even more creature info. Dungeons also has information about... well, dungeons.

Finally, every book in the series has a chapter about how you can use the things in the book in fantasy stories you create.

My niece is 4, and she can't really read much more than her name, her sister's, and a few others. She loved when my fiancee and I read Creatures to her, though. She had to ask what some words meant. Two come to mind - something was described as "earsplitting" and she needed me to break that down for her, and something was described as something like "the embodiment of chaos" or "chaos incarnate" or something. But in general, she understood it and thought it was amazing.

Suffice it to say, I am very impressed with this book series.