To eliminate the possibility of his death, check the Social Security Death Index. If he died after 1955 and you are sure of his complete name and you find him there, you can order a copy of his original Social Security application for a card which will give a lot of information. Another possibility is that he fought in Viet Nam and didn't come home alive. Search that possibility. Should be something online regarding ALL vets. If he's alive and doesn't want to be found, you're not likely to find him without laying out some bucks. Check white pages online and if he's submitted his name, address, etc. you might find him there. If all else fails, contact Social Security online and they will give you directions for writing a letter to him by sending it to them. You will need to stamp the envelope and leave it unsealed for them and include a self-addressed stamped envelope for them to send you a reply. That is how I found my missing mother after 48 years so there is hope. And good luck!!