Also, reading between the lines of parent's post, I'm wondering if the son is depressed. Maybe suicidal? Trying to "light a fire under his ass" could drive him deeper within himself, and if someone wants to die, there isn't much you could threaten them with anyway.
I certainly appreciate the destructive feedback loop: failure contributes to depression, depression and idleness cause further failure. That lifestyle probably causes obesity, making everything worse. Then there's the datelessness, etc. But breaking the cycle requires, among other things, that the kid want to get better-- I mean really want it more than he's wanted anything in his life. Short of that, and unless you're willing to toss him out on the street and hope for the best, the options are not great.
There always could be depression behind things, so one has to be careful. Maybe arrange therapy, if necessary. For a young guy to be able to open up to his father could be really hard, especially if he's in a pinch, but acting as if you were a bystander isn't the solution.
(It seems the kid has autism according to the father, so it changes things quite a bit.)
Accept it's their choice. You're probably giving them more choice than you ever have before. And it's scary for both of you. But it's literally the only way that some people can learn how to grow up.
You listen to his reality. You ask him about what games he is playing and how they work. You ask him about what he wishes for more of. You respect him and love him and build trust.
You tell him hard truths, gently, when he needs to hear them.
You accept that he will need to wander in the desert to find his own truth, and might end up somewhere other than you.
You reach back out when you miss him.
You expect him to know things you don’t, and you take joy in discovering new things about him you didn’t know were there.
You tell him when he is hurting you, draw the line on abusive behaviors. Explain why his behaviors are wrong and describe what you need from him to feel great about the relationship.
You tell him about your own struggles and weaknesses. You talk about your hopes for improvement. You are self deprecating.
You laugh at his jokes. You are playful. You try to make him laugh even if he doesn’t appreciate your humor.
You treat him the same way you treat the adults that you love and respect most, so that he grows into one.