Little Mammoth Media has an awesome video called "The Big Auto Plant." It's about 50 minutes long and shows the production process for the Mercedes M-class SUV, starting with the design process and going right through to quality control and factory test drives. (Actually all of their videos are awesome. I highly recommend them!)
I also really like DK Publishing's series of books for kids, as well as Eyewitness Books. Both lines are large (probably 9" x 12") hardcover books with tons of illustrations and photos. The information is presented in single paragraphs kind of scattered all over the page, so even though there's a huge amount of text on each page, it doesn't look intimidating. And it's highly accurate without being overly technical.
You could also check out a major bookstore if there is one near you (here we have Chapters and Indigo) and looking on the sale tables and discount section. You can usually find books like "50 fastest sports cars" or "Great cars of the 70s" for $9.99 or less.
If he's into LEGO or kit-building, you might check out LEGO's Technic line. It's a nicely complex building system (compatible with other LEGO of course) that's more accurate than regular LEGO. The cars have rack-and-pinion steering, with tiny little gears that really work! It's cool. And if you have an independent toy store near you, check it out for science kits related to automotive stuff. For mail-order, you could try www.hobbyandtoycentral.com
(full disclosure: I used to work there and it ROCKS) or www.mastermindtoys.com