I think one thing you can do is help him research the benefits of being in the military. So many benefits are there for our military personnel, but young people just don't always go looking for them. They don't read what they're given and they take a lot for granted. They also don't always map out their futures very well without help.
Sit down with him and talk about real life issues such as credit. SO many young military people decide to leave base housing and get an apartment or buy a car. They don't have a clue about compounding interest and contracts. They also don't know that businesses LOVE to sell something to our soldiers on credit because they know that the military will MAKE the soldiers pay their bill, even if the interest is up in the stratosphere, and even if the car is a lemon.
Some branches of the service will allow an enlisted person to move to officer ranks if they get their college degree (Army at least). Help him find out if that's a possibility in the branch he has chosen. Help him find an online class that will transfer to a regular university (he has to be careful not to get in a program that doesn't transfer.) This way he has flexibility to work on his studies while he is serving. It will also give him something constructive to do other than go party with the other enlisted guys during his down time.
Talk to him about appropriate sexual behavior and ways to say no to what he's going to see others do. Teach him about how to prevent pregnancy and not allowing his partner to handle all the details of birth control. He needs to use condoms every single time even if she is using birth control. You will have to understand that pretending that he won't become "a man" is not going to help him.
Lastly, though this sounds kind of strange, try to think things that he might misunderstand. Young people sometimes just don't have the experience to think some things through. An example I'll give you is a family member who went into the service. She thought that military drivers were exempt from the rules for driving in the HOV lane. When the policeman pulled her over and she told him she didn't have to have 3 people in the car because she was military, he made her get out of the car and walk all the way down to the sign (it was quite a ways too) and read it out loud to her. He asked her "Does it say anything about military on that sign?" She was pissed, but she learned the lesson.
How do I know about these things I'm telling you about? That same kid with the HOV story made mistake after mistake with her credit, buying a lemon car over 20% interest, same thing with furniture that broke because of her friends sleeping on it (long before it was paid off) and all other kinds of messes. She also had a baby by her douchebag CO. I'll never forget talking to her about it and her saying "But I luv him!!!" The military took out child support from his paycheck for the rest of the time he was in the service. If your son makes the mistake of not being careful, he'll be in the same boat.
I know you are excited and proud of your son. I'm proud for you both. Both me and my husband have military service in our families. Other posters are giving you different kinds of advice. I offer this not to put a damper on your joy, but to help you help him have a more successful time during his military tour.