When your son becomes licensed most insurance companies will require you to place him on your auto insurance policy as a driver since he will likely drive your vehicle and thus require coverage.
The Texas Department of Insurance Auto Insurance Guide backs up this idea. Here it says young drivers must comply with the state's financial responsibility laws, just as older drivers do. Most young drivers, however, have the option of satisfying their legal requirements by being added to their parents' auto policy. Adding a young driver to a parents' policy can be expensive, but it's cheaper than taking out a separate auto policy.
A parents' policy covers children living at home or away at school, even when not named on the policy. Even though children are automatically covered on their parents' policy, it's important that they be listed on the policy as soon as they reach driving age. Insurance companies are required to charge the correct rate, based on the classifications of the drivers in your family. If you do not have all of the drivers in your family listed on your policy and the company learns about them later - because of an accident claim, for instance - the company will bill you for the extra premium you should have paid.
The Texas Department of Insurance site does not state specific insurance laws here but you can contact them to get more information and where in the Texas law you can read them.
The TX DOI also notes that you can sometimes remove a teen-aged driver from your policy by purchasing a non-owner policy. However, this usually is a bad idea. A non-owner policy merely provides additional liability insurance when driving a non-owned vehicle. If your teenager has an accident while driving your car, neither your policy nor the non-owner policy will pay for your vehicle's damage. You might also be unprotected financially if held liable for an accident caused by your minor child. Finally, if the non-owner policy is rated properly, your teenager's liability insurance might cost as much as or more than if he or she was on your policy.
So hopefully this information will show you the reason for needing to add your son to your policy though he does not own a car. If he is not going to drive your vehicle at all then you may see if you could exclude him from the policy but this would mean that if he ever did drive your car that there would be no insurance coverage extended to him at all. If he gets his own car with its own policy then at that time you could show proof of that policy to your insurer and take him off of your policy.