It is highly unlikely that your son will be able to drop (cancel) his car insurance on this car even if he is being deployed and will not be driving it. When you have a lien holder on your vehicle, due to a lease or loan, they require certain car insurance coverages to be kept on the vehicle - whether the car is being used or not. Normally the auto insurance a financing company will require to be maintained on their asset, the vehicle, is what is referred to as full coverage. Full coverage is what most people say you have when you obtain at least the state minimum Liability coverages and physical damage coverages of Collision and Comprehensive.
Your son can contact his lien holder to discuss his situation of being in the military and going to be deployed in March with the car parked at your house to see what specific coverage they require him keep on the vehicle during the time of his deployment. Likely they will say that even if the car is only going to be sitting in your driveway that full coverage will need to remain on the vehicle. Comprehensive is something especially important if the car is just sitting since it may make the vehicle a prime target for theft.
If the lien holder actually does allow your son to take auto insurance off then your son would need to speak to your state's Department of Motor Vehicles (DMV) about the registration of his vehicle. Most states require as long as your vehicle is registered it must have car insurance maintained on it. If he can take insurance off of it then likely he will need to turn in his registration and plates. In some states, such as California, if you are not going to use a vehicle you can sign up with the DMV for a planned non-operational (PNO) registration. With this you can take insurance off of your car while the vehicle is not being used and then re-register your vehicle when it will be used and have re-insured it.
Another thing to think about is what your state and local government say about a car being parked in your driveway without insurance (or registration since without insurance likely he would need to turn in his registration and plates). Many state or local laws say that a vehicle without insurance and registration may not be parked where it can be thought of as an eyesore. Even if there are not laws in your area your own homeowner's association (HOA) may not allow you to park your son's car in your driveway if it will not be used for a long period of time.
Once your son finds out if he has to keep his auto insurance on his vehicle or not from his lien holder then he can speak to his car insurance company about his situation. If he must keep his insurance on the car he may see if his automobile insurance company has any discounts that could apply and thus lower his rates while he is deployed. He can ask about any discounts for service members in general and specifically about military personnel that is being deployed and their vehicle will not be used for whole period of their deployment. Even if his car insurance company does not have discounts for military members his rates may go down since without the vehicle being used his annual mileage will be lower and thus may lower his rates.
Before your son deploys may be a good time for him to do some car insurance comparison so that he knows he is getting the best rates possible on his vehicle while he is gone. CarInsurance.com can help him get quick quotes. We can help him check car insurance prices and find the best car insurance for his situation.