Boys might start out playing with Matchbox cars, graduate to radio-controlled trucks and then move on to the real deal. If your son shows an interest in automobiles and mechanics, why not show him around the family car's engine?
Get out your vehicle's manual as a visual aid, and teach him about the different parts beneath the hood and how they work together. If you can, elevate the car for a peek at what's on the underside of the automobile, too. Educating your son on how to change a tire and check the oil and transmission fluid levels and other basic car care techniques will be useful throughout his life.