Do you have to put your step son on your auto insurance if his on his grandpas?

If he lives in your household he must be listed on your policy. If he never drives any of the vehicles in your house the insurance company may allow you to sign an exclusion so that he does not effect your rates. The downside to this is that an exclusion means that no coverage is provided if he is driving any of your vehicles and a claim occurs. This does not mean he can drive if it's and emergency or if his is disabled. It means he cannot drive your vehicles period. Keep this in mind.