First time posting here. Hope this is the right place.

I am looking for some help/advice/insight. 

So my parents and I think my 29 yo brother is suffering from undiagnosed and untreated mental illness. He was a perfectly happy kid and then hit a rebellious streak in his teens and and started a dark drift downwards in his 20s. Smoking pot and drinking turned into abusing coke and serious outbursts and bar fights. And jail time. He ran with a rough crowd and has eluded to my mom about doing some heinous things . He's been in a few 72 holds but also checks himself out and refuses to try any treatment for mental illness. 

So I'm sure this just sounds like the ravings and actions of a drug addict/alcoholic but the last few years he has gotten progressively worse. He was living with my parents in a pretty stable environment and wasn't free to abuse drugs and alcohol. He has threaten and tried to commit suicide multiple times. 

He began getting "spooky" (as my mom describes it) in the last few years. He thinks people are out to get him. Up all night with paranoia. Pacing the house, locking doors. He was adamant people were under the house one night and doused the outside with gasoline, with my parents sleeping inside. Managed to get the gas all over himself and in his eyes and came in screaming and raving awaking my parents. I hate to imagine what might have happened if he started a fire.

About a year in a half ago he ran off and shortly after would up with some very serious charges and did 9 months in jail. Plenty of time to clean up and detox right? Gets out, seems normal enough. Comes back to my parents to live. Starts drinking bleach for some reason. Thinks he is being targeted by the government and CAI with radio waves. Watches a lot of YouTube about these sort of things. I video chatted with him less than a week ago and he really believes this sort of stuff. We talked for a few hours and he wasn't drinking or doing drugs in that period. 

So my questions are what can we do? Has he abused so many drugs that he is permanently damaged? Is that treatable? Is he undiagnosed schizophrenic and has been self medicating?  Does anyone have advice or similar situations? All advice is much appreciated. Thank you.