Unfortunately, you're dealing with an "adult"; my guess is that this behavior is not suddenly onset. "Notice the early indicators and stop the behavior before it starts," is not helpful at this point, though it would be the most effective.
Keep in mind that, if he's as argumentative and defensive as he seems, that if this conversation were to happen in person, it may devolve into arguing and become fruitless; if things seem to be going that way, perhaps it would be best to cover the important matters in smaller chunks or, if things truly hit a roadblock, communicate in writing, either in email or through a letter, which will hopefully allow him to read the entire document to get the full understanding of what is trying to be conveyed before having an emotional reaction.
Understand that there is a reason he doesn't want to try. That reason might be a disorder, or it might not be. Either way, it's probably not something that can be solved in one "intervention".
Begin from a place of love, without seeming condescending or holier-than-thou. Express concern for his future, without sounding clichéd. He needs to understand that his parents are concerned for his well-being AND that they might not know everything about his situation, but are willing to listen and/or get him the help he needs, if he wants it (some people don't want help, and trying to help will only make the situation worse; I have no idea if he's one of these people).
Your profile puts you in the United States; I can only assume that the family involved is from the US as well. In my experience, US society has been sold on the fact that going to college is the only way to be successful and happy; this is just not true. While I have a college degree and a job in a field that requires it, my brother graduated high school early, has no college degree, and has a job that makes him financially better off than I am. Purely anecdotal, but my point is clear. Perhaps he'd be better suited for a trade, like welding or plumbing. Perhaps he'd be successful in a branch of the military. Attempt to have a discussion about what he would like to do. If the answer is "nothing" and/or "smoke pot", there may not be much you can do. If he wants to become a musician or an actor or some similar profession, attempt to convey the difficulty of the fields while trying not to tell him no; if it's not meant to happen, something/someone else will tell him no. If he gives any other answer that has at least some merit, discuss it. See if he has any particular plans, ideas, or dreams. Tread lightly in the realm of telling him how he should go about accomplishing his goal. Perhaps do some research, and in future discussions discuss results of possibilities.
If the parents want to switch from the carrot to the stick approach, then setting a timeline with a series of expectations may be in order. That timeline could even be on the long side; just something that is set in "stone" that carries with it consequences.
Or, keep making empty threats and he'll learn that he can get away with doing whatever he wants.
This puts is a little stronger than I would, but it's true. Let's say that Person A tells Person B that if X happens, then Y will be the result. If X happens, and Y does not follow as was explained to Person B, then Person B has no reason to believe anything that Person A says, and regaining credibility is a LOT more difficult than retaining it.
Finally, if the lack of motivation does not stem from some sort of disorder: people may have different motivations for wanting to succeed (succeed being defined VERY broadly here), but a lot of it comes down to fear. I fear that I won't be able to provide for my family, so I want to get a good job. I fear that I might screw up my kids, so I try to become a better father. For someone with no fear, it is likely that they have no drive. That fear needs to be internal (I want to be better person) or external (I want to eat). The unfortunate truth is that he may need to experience some pain in his life to help motivate him to rise to the occasion, the occasion being his life.
