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.)