Very hard. DoD doesn't like to release next of kin info because dirtbags call and harrass them. HOWEVER, go to military.com and see if you can find his unit and search for his buddies. Or search for him on their database and find a home state, home of record. Might help. Good luck.
Try going to www.rootsweb.com It is the largest genealogy site in the world and see if he is listed and follow the descendants.