Is he possibly Autistic OP? A slightly familiar pattern in your posts so I am only putting it out there.
Co morbid conditions are much more common. The smoking weed is often a self medicating behaviour.
The life falling apart could be burn out. The black and white thinking blaming you for everything.
This is a little familiar to me hence asking.
You can pay for him to see a private psychiatrist if you can afford it. It's going to be tough to encourage him I imagine.
No one is going to scoop him up and give him a thorough once over if he isn't willing. You can hardly get anyone sectioned anymore. Kind of glad for the poor people who need love not imprisonment but it does mean in vsdrs like this it's usually desperately searching yourself for someone's to see him quickly.
I believe many ND people get screwed over by getting some random diagnosis they don't have with our someone properly assessing for potential Autism and then additional mental health struggles.
The Paranoia could be the cannabis alone and I appreciate ND isn't an explanation for every troubled person. I had a neighbour who lost it entirely after growing weed and relentlessly smoking it.
People do listen to conversations though just to add. I had my own recent situation thinking I was going insane over a suspected hack. People can access your phones emtire interface now, even see through your camera, someone can link into your WhatsApp from another phone ( you can detect it if you check).
If he's seeing people in his room listening then it's at another level of course.
I'd check what he means re the conversations.