It really depends on your current orders.  If the current orders have a provision for what the visitation schedule will be if he moves farther away, then you just need to switch to that part of the orders.  If your orders do not have a provision for what happens when one parent moves off, and if you and your ex cannot reach an agreement on what to do, then you may have to go back to court to seek a modification of the custody arrangment.  You would need to do so before he is deployed to insure that you can have a hearing before he is shipped off and is unavailable.