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Thank you for clarifying.
There are limited options to involuntarily commit someone to a mental health/hospital. 1/ the individual must pose imminent danger to self or another or 2/ family members can try to get a judge to sign a lay affidavit (by sharing their concern about this person's mental/medical status.)
The judge once he signs the lay affidavit, the sheriff dept can pick him up and bring to a crisis center where he can be kept for up to 72 h. Of course, this is just for crisis stabilization purposes (he could chose to leave after 72h and also can chose to decline medication while there) You may want to contact someone for clarification of the process (versus from state to state)
You may try a family intervention (but if he is in a paranoid state) this may yield littler results.
Psychosis (paranoia) can be triggered in those with severe depression or those who may have a diagnosis of schizophrenia. It is managed by mood stabilizers and anti psychotic meds.
It sounds like schizophrenia (symptoms start to manifest in the early 20's) The chances are higher if there is history of it in the family.
Other possibilities are if he is severely depressed or abusing drugs.
The symptoms listed does not sound like bipolar where a person shifts from depression to mania (and bipolar disorder would have manifested earlier in his life)
The marijuana is not a hallucinogenic, so most likely this is psychologically based.
IS there anyone that he trusts? That person can persuade him to see a psychiatrist as medication is very effective in managing delusions.
The options then would be limited (unless he poses a threat or is violent) involuntary commitment would be hard.
As far as "reasoning "with him, you would not be able to reason when he is in the paranoid stage. If there are ever days that he is less paranoid would be the time to talk to him. You would want to share your concern and let him share if he will, how you as a family can help him feel better. Trying to get him to trust you would be a priority. You would not be trying to persuade him that his thoughts are delusional. Rather, you would be providing some support by asking him what this sort of thinking is causing him to feel (anxiety, frustration, depression, etc) Then, educate him on the treatment options (of course, you may not want to do all of this in the same day) Each day one step at a time.
Once you gain the trust, then you could challenge his thinking process as to where he can see that his thoughts and behavior are alienating others from him. You can point out that if he is distressed, there are things to make him feel better as long as he is willing to try them.
The Complete Family Guide to Schizophrenia: Helping Your Loved One Get the Most Out of Life by Kim T. Mueser PhD and Susan Gingerich MSW (Paperback - May 26 2006)
The approach would depend on what evokes a calming response in him.
It may be best not to engage him when angry (perhaps say, let's talk when you're calmer, or we can talk about this when you're feeling in a good space, etc) It would be important not to escalate your voice/keeping it calm. if you have to remove yourself from the setting though, you would have to do what is safe and better for you (sometimes you can not reason with someone who is having a fit. It may escalate them)