You can continue to give him advice and encourage him to set some short term goals. Focus on what he is willing to do at this time in his life, and point out to him that to obtain certain goals, he would need to apply himself.
If you believe that his behavior may be due to underlined depression, (individuals who are depressed may lose interest in things that were of importance or were pleasurable to them), suggest that he sees someone.
If you can find motivational workshops in your area, let him know and encourage him to go (he can always consult with a career counselor) There are on line career tests available as well.
The issue you're struggling with is how to let him see your point of view. He may in fact hear what you're saying but is not interested in making changes. He must want to change (no matter what you or anyone else says; it has to be his choice)
If he relies on you for financial support or resides at home, see about him getting his own place and income to support himself. Otherwise he won't be motivated.