Click on Member list on the top of the page. On right had side is a space that says search member with a place to enter names. Don't, look just below and click on advanced search. There are several fields that show up, and put in what ones you know, then click search.
Most members here don't post their real names so doing a search may not be productive. Why don't you just mention the members name and maybe he will see the post or someone will know how to find him.