Not a doctor but have spent some time in psych wards. Sounds like psychotic depression - normal and treatable but he needs to see someone soon. It’s easy to get sucked into someone’s delusions - they are usually VERY convincing to the point of questioning your own rational thinking. I suggest focusing on gentle suggestions of getting; these people need support not someone buying into their theories. It’s tough and heartbreaking but you need to be the strong, older one here and do the right thing which is get him to a doctor or straight to the hospital

It’s really REALLY hard to get them into see someone but the brain will go red eventually. The faster you can get him to help the better