Yeah, classmates.com is a pretty good source, especially if you know the high school your friend attended and the year he graduated. Even if he isn't registered, you may be able to contact one of his classmates who can pass a message to him.
Another good way to find your buddy is a book called "How to Find Anyone Who Has Served in the Military" or something like that. The appendix has a number of resources for locating veterans and serving personnel. That book, or others like it, are usually available at your public library.
Magician, Sailor, Adventurer...Been There, Done That