What's his incentive to leave? He has full financial support and a nice place to live, rent free.

But if you provide an incentive for him to become more responsible, he will seize it. I would continue to allow him to live at home but provide absolutely no spending money for him whatsoever.

He'll get tired of never having any money to do what his peers are doing and will probably breach the topic of employment himself. Once he gains employment and see the financial and self-esteem benefits of working, the positive momentum will carry him all the way out of your house and into adulthood.