Kyle had an address within 6 weeks of arriving in Iraq, last time. This time, he gave me an address before he left. A week later he gave me a new one.
Usually, they know their address before they leave. The problem is that it may change during the first couple weeks he's there. He will still, probably, get anything you send, but it might be a little delayed.
I wouldn't send anything to the base to be forwarded... I doubt it would go anywhere at all. I'd just wait until he calls you with a firm address that he's been given.
Have you heard about motomail? Let me know when you have an address for him, and I can show you how to send him motomails. They are free, and faster than regular mail. Good luck.