START WITH WHAT YOU KNOW
You know his name? Previous phone numbers? Address where he used to live? If you think the information you currently know about him is not enough to locate that person you're wrong. Possibly, see if you can find more data on him but for now start with what you currently have.
CHECK IF HE HAS A SOCIAL NETWORKING PRESENCE
Social networking = sites like Facebook, Hi5, MySpace and so on. They all have search engines that can help you locate people (in this case, your classmate.) If you don't want to search through all of the social networks separately then use the following websites to search through all of them at once: Wink, Spock and Pipl (the links are below on this page in the resources section.)
SEARCH BY SCHOOL
You don't have to search for the individual only. Try to search by school. I am very sure you'll find other classmates from the same generation. Contact them and ask if they possibly know something about the person you're looking for.
Check for address, phone number changes, records. You can do this by ordering a background report about him. There are lots of companies to do this. I recommend you go with USSearch (the link is below under "Resources") because it is the biggest and most popular company on the internet on doing individual background checks.