Good luck to your son in his military career. Yes, you usually should be able to drop your son from your car insurance policy since he will be away from your household (and your vehicles) for such a long period of time.
Although it's likely your son will keep your home address as his permanent address for the next four years, you should be able to take him off your auto policy as a driver unless he is located near your household and will be visiting regularly or driving one of your cars during his active duty time in the military.
With your son living away from home and not having access to your vehicles on a frequent basis, he isn't a risk to your car insurance company. They should allow you to remove him as a driver during this time.
If instead your son's military base is close to your home (e.g. within 100 miles) and he is going driving one of your cars during his boot camp or time in active duty, then you would normally need to make sure he still had coverage.
Each car insurance provider has its own rules, however, so before taking your son off your policy, speak to your agent.
If you plan on your son coming home to visit every so often and may drive your car during these times, also mention this to your agent to make sure he would be covered, even if not listed as a driver.
If your son ends up buying his own vehicle while away in the military, then he should get his own car insurance policy.
When shopping for his own insurance, you son should ask about car insurance discounts. Many auto insurance carriers offer military discounts; for instance, Geico offers a discount of up to 15 percent off your total insurance premium if you are on active duty, are a member of the National Guard or Reserves or are retired from the military.