When my son was in Iraq we were told that OneSource was the best first place to start with virtually any issue we had involving my son and his deployment. They would be the best place to start. If they can't find him for you they should be able to put you in touch with someone who can. It would probably help if your mother or father were the ones to contact them since a parent will carry more weight then a sibling. You will likely need his social security number to track him but your parents should have that.

I don't know the details of your situation but be prepared for the possibility that he will not want to be contacted and they won't give out his information if he request that they don't.