That's a great idea. I bought his brother an anatomy encyclopedia, and he loved it. The Knowledge Encyclopedia looks pretty good - pictures, short explanations.
Deal link: Amazon
It's a nice choice for the kids to satisfy the curiosity and learn something at home.
Not bad price.
Free shipping for Amazon Prime or order over $25.
There are some used - good condition less than or around $10 including shipping.
Book: DK Knowledge Encyclopedia (Updated and Enlarged Edition): The World as You've Never Seen It Before
Deal link: Amazon (Download iOS/Android App for early access to deals)