I've searched all the people search sites and can't find my 18 year old son can you tell me what else I can do he lives with his father and I know his name and address but still can't find my son

Have you hired a private investigator yet? Talked to the fathers family as to where they might be?

If you know his address it shouldn't be hard to find his phone number and connect with him by phone, or write him a letter and send it to the address you have.