This post is about my best friend, we'll call him Tom, we've been friends for a long time and he's been normal all throughout his life (we're both 19)

The last couple of months he's been cutting off more and more friends and is becoming more reclusive. He says I'm the only guy he really trusts now apart from his family. 

It's hard to summarize whats going on, but I'll do my best: 

His view:

He has had a crush on this girl a long time, the ex of this girl, we'll call him Brian was also friends with Tom for a long time. The last couple of months Tom started hating Brian more and more, he thinks Brians step dad has a child molestation ring and he thinks they want to kill Tom because he knows about this. He's been making up crazy conspiracy-like theories (Brian having spiked his drink at a party, another friend trying to set him up for being raped, Brian's step dad bribing Tom's driving instructor because his driving instructor "acts weird", people following him etc. Honestly way to much to state here) He's been sending his theories to me over text, and spends an absurd amount of time every day doing this (I'm talking 1,5 hours per day at least).  He even stopped talking to one of our mutual best friends cause he still talks to Brian. 

He was never like this, the most worrying thing is that Tom's parent's support these thoughts to a certain extent. I think they "support" it because they're scared of talking back, Tom has always been somewhat of a disrespectful child towards his parents. 

Tom also smoked weed every single day and drinked sometimes when he was home alone too.

I'm honestly kind of lost, I don't know what to do and I don't want to lose one of my best friends.

Any kind of advice is appreciated.