My close friend has been having, what I believe are, severe delusions. It started a few months ago where he suspected someone was breaking into his apartment. His clues were very mundane, like a window being closed when he never closes the window. Then, I guess while trying to figure out what was going on, it took shape. He began suspecting our group of friends (we've been friends for 10+ years) of trying to steal his identity. He ended up suspecting everyone except for me (we used to be in a relationship, so he trusts me). During the identity theft phase, he installed cameras and an alarm system to try and catch whoever was, at this point, repetitively breaking and entering.

I became a reliable person in his eyes. I started checking up on him, and slowly tried to debunk his theories in hopes to convince him to seek professional help. That attempt somewhat failed... he did end up realizing that his friends had not in fact conspired to steal his identity... but his new theory is worse. 

He has now begun believing that his boss has hired his neighbors, friends, colleagues and a private investigators to watch him and attempt to incriminate him in order to fire him. He has an entry level position at a corporate job and thinks their motive is his alcohol and weed consumption, and the fact that his boss doesn't like him. His story is that they stole his passport when it all started to distract him from the bigger coup. 

I asked for proof and all he had to show me was a video of a suspect trying to get into his apartment. The video ends before the suspect leaves the premises. I described the said "suspect" to another friend and it seems to be his neighbor with whom he regularly hangs out.

Obviously as with any psychosis, there is no reasoning with him. He is also very convincing and seems lucid other than the fact that none of his theories make any sense. 

I talked to him about psychosis and persucatory delusions and he understood where I was coming from, but promised me that his mental health is fine.

What can we do ? He does not want professional help. 

PS - his family has a history of bipolar disorders and his mom has huntington's disease.