Strawberry House Mouse, you said "I honestly don't know what his reasons are" and I think that is the real problem. You need to talk to him about his feelings and motivations and really listen. As a warning, you may hear some things that you don't want to hear or are not prepared to hear. He may not tell you his real reasons right away, so be prepare to ask smart questions and listen carefully to his answers to follow through and get to the truth.
People suddenly decide to join the military for a number of reasons, but in my opinion when a person reaches this decision without previous interest in the military that person is usually trying to escape from something in his civilian life.
Is he unhappy with his job? Has he recently been passed over for a promotion? Have a number of his friends joined the military? If he trying to compete with a family member or friend who is doing well in the services? Has he always had an interested in the military but you didn't know of his interest? Could he be unhappy in your marriage and be trying to force your hand to initiated conflict? How long have you been married and are your children his children? Could he feel trapped?
I hope I'm not out of line with these questions, but I have a gut feeling that his motivations are very important in this decision. If he enlisted, it is not just a career change for him but also for you, so it is a decision you should try to make together and you can't make decisions together unless you are really talking honestly.
Good Luck!