Welcome to the world of the military.
Yes, they do tell you that you should be safe or get a deferment but when they need bodies they pull bodies to fill the slots. You knew that this could possibly happen when he joined the National Guard.
Now you will be part of the other society the military one. Contact a local unit for support for spouses and parents and get information on what you can do while he is away. They will have lists of things you can and cannot send to him while away. Create a support group if there is not one nearby so that you can all cope with the deployment together.
Thanking your son for his service to his country and your brave support behind the scenes.
The other S.
Retired Military Wife
PS This will give a time to reflect and to mature into the man he wants to be.