Your concern is valid as his behavior is indicative of paranoia. If you are aware that he may have a preexistent diagnosis of either major depressive disorder or schizophrenia or if you believe that he may be abusing drugs, you could only encourage him to seek help.
Perhaps try to find out if he had ever been on any psychotropic medications or if he had ever been evaluated by a mental health professional.
You can only encourage him to seek help but ultimately he has to do it himself.
Sometime when people are not well or are irrational they do not think to seek help so if he is open, you could offer to go to the doctor's visit with him. He ought to be seen by a psychiatrist.
Depending on where you reside, you may want to find out how to seek mental health for him if he is not willing to do it on his own and if you are concerned about his welfare or that of others. If a person is a danger to self or others, they may be involuntarily held against their will for a mental health assessment for up to 72 hours.