First he will have to use the seat belt, and, when he is tall enough, the shoulder harness as well. Don't use the shoulder harness until he is at least four feet tall, so that it rides across his shoulder rather than his neck. If your child is shorter than four feet, put the shoulder harness behind his back and use the lap belt only. This shows up in several ways. Boys, for example, think about and pretend that they are the mates of their mothers, and might even imitate the swagger of the young man showing off for the girls.
Girls think about being the mates of their fathers.
These fantasies are a normal part of the child's development at this age. They come with a growing interest in sexuality, such as the difference between the sexes, and where babies come from. Children should not be shamed or feel put down for asking such questions. In fact, when your child is older and it is time to speak to him about sexual matters, it will be much easier for all concerned if there has been an open attitude about the body and its functions all along. Along with your son's increased interest in sexuality, he will probably also play with his own genitals and may even be interested in the genitals of other children.