This man is clearly developmentally disabled and has lived his life without diagnosis or help. Even my sister who is on the spectrum who can't drive and needs help with most basic things can cook a grilled cheese sandwich. Im not gonna judge you for how you reacted but I think his parents dropped the ball at some point in his life.

Maybe I'm wrong, I'm sure you know him well enough and he really is just half man half slug; but this is the most ridiculous case of incompetence/laziness if that's all it is.

For some things cleaning up after yourself some men just don't think about that shit. But from your description this man is barely functioning, it goes beyond just having a brain wired different. Does he even have a job/family/friends? It's not your responsibility to care for him you have your kids.

But he might just be autistic and you might be the only person who could help set him up with the help he needs. Dshs should be your inquiry if this resounds at all with him, autistic people have it hard and can become homeless transients very easily.