I'm sorry to say this, but the tone in your voice towards your son sounds more detached than it should in my opinion. I mean, why would you let him slip into that bleak future you're describing for him? Shouldn't you be the one to wake him up, especially if he lives under your roof.
I get it if you don't believe in discipline, but I think that's what is required here (or at least fatherly guidance).
I'm usually the biggest advocate for personal responsibility, reaping what you sow, etc. but I think you may be being too hard on the parent. For one thing, parents lose a lot of power as kids age-- how exactly do you punish a 25 year old living at home? Send him to bed without dinner? Take away his Nintendo Switch?
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.
A caring father can hurt the child's development by making him feel too comfortable. You can show that you care by putting boundaries. I'm not sure the age matters here. You can cut down the internet, or make him pay rent. You can be hard, and at the same time give him options. Show him somehow that the reality could hit him in the face really hard, if he doesn't change.
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.)
Kick him out. Having my parents cut off financial support to me was the kick in the ass I needed to get into shape. Best thing that ever happened to me.
You talk to him. Drag him out to sit in the yard and talk for an hour. Demand he ride with you to the grocery store.
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.
Tough love is sometimes the best form of love. Be prepared to be resented; be prepared for the child to cut you out of their life.
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.
