If it's a planned disappearance then his computer might well yield some information. Use a tool to extract web history from deleted file space (and live space)

If it's a planned disappearance then his computer might well yield some information. Use a tool to extract web history from deleted file space (and live space) and have a go through it - if you have access. If he visits forums, blogs etc. ask for their help in obtaining the last IP address they have for him - this would track him to a rough geographical location.