How does one assist an "adult" child to become a functioning adult?

One that has been financially supported by his parents his whole life, is thirty years old and still aimless. Cannot seem to hold down a job, and currently is living “off” his girlfriend.

He has many qualifications, and is capable. What is stopping him?

Aside from a full mental evaluation, what types of practical things could help? What is a positive way to say “No” to this person. In terms of loans, never paid back or money or constant emotional support. I am looking for positive ways to deal with this. Since I suspect also that, confidence or self belief is part of the issue to start with. What would one say to the girl friend of this person, who has basically said, if he doesn’t start to earn their days are numbered. What types of contributions are expected from partners? Sometimes this does not involve money.