His paranoia sounds like psychosis which may be weed-induced.
He absolutely has to stop using weed or any drugs for that matter.
Could he do something positive like taking a course in martial arts? Learning a musical instrument...landscaping/gardening something concrete and tactile to ground him.
Is it possible to speak with a psychiatrist to discuss his case and possibly have him admitted to hospital? Depending on the diagnosis he might be eligible for some type of disability pension.
It's very worrisome, the state he is in...