First off, the definition of a household member for auto insurance companies can vary from one insurance company to another. In general though, immediate or extended family that lives with you as well as non-family household members that drive your vehicle, such as a roommate can be considered a member of your household.
Typically, family members include any immediate or extended family member of driving age. This Includes parents, children, siblings, grandparents, aunts, uncles, cousins, foster children, in-laws and step family members. There may be some variations in this list among different carriers as well.
"Household" residents that generally do not need to be listed include non-family household residents who do not drive the vehicle, immediate dormitory roommates who are non-family and do not drive the vehicle and additional residents of a military barracks need not be listed as drivers.
For coverage purposes, all family members of driving age who are residents of the household must be listed to protect both you and the insurance company providing you with coverage. In some states, exclusions are available for family household residents who do not require coverage.
Insurance companies' guidelines differ so you will need to discuss your son being away at college with an insurance agent from your specific insurance provider. Most insurance companies require that you include all household members on your policy, including any children away from home or at college that may operate your vehicle during the occasional trips home.
If your child will be attending a college at least 100 miles away from home and is not taking a car, you may be able to get a discounted rate for them; this again differs depending upon the insurance company's guidelines.
The California Department of Insurance notes that if your child is away at school (college or university) and is not taking a car with him or her, some companies will offer you an exclusion endorsement that basically says I know if he gets behind the wheel of a car, he is not covered. In this case, there's no charge for insurance, because there is no coverage, but the child would also not be covered to drive your vehicle when he came home for visits.
Comment Update: The reason the person needs to be listed is because the insurance company may be exposed to any costs that arise from actions of any household member. State laws differ, but typically the liability exposure is not limited to just the owner. You can exclude them, but the company still needs to be aware of them to include or exclude them from your policy.