First of all, how old would this person be? Is there a possibility he is no longer alive? I did a search for our chapter's local founder (1926) several years ago, only to find out that he had already passed away. Fortunately I was able to find a brother who had gone to school with him.
If you are reasonably certain he is still alive, find out when and where his last known address was. The more information you have the better. There are several ways you can begin to search for someone.
While I am not adopted, I encountered someone several years ago who thought my mother might have been his birth mother. She wasn't, but we were able to help put him in touch with the right people (same last name, same home town, different families). He told me a lot about how he conducted his search.
If you have a social security number, your process should be fairly easy. There are online "missing persons" sites like peoplesearch.com, that can help you locate someone. They have been featured on shows like Montel, Oprah, etc. Usually with a name and social security number they can find someone within a few hours.
If you can't get a ss#, if you have his last known address, or even his hometown, you might be able to locate family members who would know his location. If you can, visit or call the library in his hometown (providing there is one). If the library has city directories, you should be able to find out when he left that address and if there are still relatives in that town. Depending on the last name, it might be fairly easy to track down some relation who would know his wearabouts.
Records at the city hall or courthouse (marriage licenses, birth certificates, deeds, wills, tax records,etc.)and local newspaper archives are also good sources of information. I once heard something that the most effective way to locate someone is through the DMV database of driver's licenses, and I believe this is what many private detectives and police agencies do.
Anyway, good luck with your search and let us know your results!