Magazine for nephew
Just get him a few subscriptions. Even though there is a good bit over his head, it's a starting point of finding out a ton of things. He will have to find someone to ask questions to. Friends, relatives, school teachers. If he is lucky enough to get to a track and find a guy willing to answer a few questions he will be in good shape. This, is how I started.
I also had the good fortune of a few good local repair shops that had racers as owners and mechanics that didn't mind answering questions. I did manage to buy them from time to time until there sense of humor and work load had them kicking me out until a better day arrived.
The best learning tool I had ever gotten my hands on was the MP engines, chassis and catalog book. It took a ton of mystery out and added a ton of knowledge in. And this only lead me to tinker more and more. There was no internet. But since there is now, he can read here and else where. Just fore warn him that all the info he reads is subject to not being accurate and/or there is a better way. Or cheaper way, which may not be wise, better or other.