Put in a search online for his name and any names of close relatives. It is amazing how much you can get free on the Internet. There are companies that will try to get you to send payment but you can do it yourself.
If you know what state or better yet city you can sometimes find public records through government offices. If he owns property it will be easier to track him down.
Sometimes you'll find news articles, Facebook pages of nieces or cousins that know how to get in touch.
Keep thinking of any family members you can recall and try those names too.