In general, most insurance companies want to know who resides in your household. From there most insurance companies will want you to either list that person as either 'eligible' to drive or 'excluded' from driving. If you want your son to be covered by your insurance policy you will need to inform your insurance company when he obtains his license (or permit) and most likely they will require you to place him as an eligible driver on the policy.
It is not necessarily a state law that requires you to add your son to your policy since you are the main custodian parent of your son but likely a requirement by the insurance company because of their internal underwriting guidelines. Most state insurance regulators allow insurance companies to consider all resident operators of an insured vehicle in rating of an auto insurance policy. This includes your child, even if he only a learner's permit or has just obtained a driver's license. To find out what the Utah Department of Insurance permits you can contact them directly.
Now when teenagers split time between divorced parents it is a common question of which parent should include the teen on his or her auto insurance policy. Many insurance providers suggest the parent, who has custody of the teen the most, should add the child to his or her policy. In some cases, insurance companies say it is whichever parent has custody of the teen when the child is attending school. It is a simple question to both insurance companies, so we suggest you ask their specific guidelines.
If the same company insures both parents, the teen will normally be covered by both parents' policies, regardless of whether the teen is listed as a driver on either policy. That is because some insurance policies define "an insured" as a person related to you by blood, marriage, or adoption who is a resident of your household. Since not all policies are the same though you will need to read through your policy language and/or contact your insurance company for details.
If you are required by your insurance company to list your teen on your policy and you do not, be prepared for the potential consequences. Cancellation, non-renewal, denial of claims, and surcharges are among the consequences you might face. Some insurance companies will cancel your policy for misrepresentation, if you fail to list your teen driver during the application process, or when the teen gets a driver's license.
Some insurers require all drivers be named on both policies, no matter how much time they spend in a household. If your teen has access to your car and drives it even occasionally, you might be required to list the teen on your auto policy. So your son's father may also be required to place him on his car insurance if your son will drive his vehicle during his weekend visits. His father thus should also check with his auto insurer to see what they require him to do one your son gets his Utah driver's license.