Adult Children

How do you motivate an adult child to do something? My eldest is not going to school, is not working, is not volunteering, he stays home and plays video games... I have tried everything I can think of to get him to do something, anything... At what point do you say - enough. I don't think he would be successful out on his own at this point. He lacks in self confidence and in social skills - but the longer he stays home - the deeper his hole becomes. He is so gifted and talented, I find it frustrating that he isn't applying his assets. Gosh sometimes this parenting thing just sucks!