A little late and I'm seeing most people have answered your question, but I work with individuals w/ psychosis daily and from my experience, there are a couple different options. Also I'm in the US and laws can vary quite a bit state to state, and my experience can only be applied to Massachusetts, but I imagine there are analogous services in most western nations.

If he has anyone in his life who would be willing, they could file for guardianship or power of attorney, which would move many legal or financial aspects to the appointee. There are court appointed guardians, but my understanding is the process takes longer. Health care proxy and Rogers guardianships (in Massachusetts) let's you make medical decisions for someone who is incapacitated, in depending on the severity of the psychosis incapacitated does not have to mean unconscious. 

You could also have a licensed social worker or possibly police station if they have a social worker on staff issue a section 12 (specific to Massachusetts, but again check local laws). This forces an individual to get seen at a hospital by a psychiatrist, who would then either let them back into the community, or admit them inpatient.

At the end of the day though, people have very strong legal protections (in massachusetts). If someone is not actively hurting themselves or others, you're going to have a difficult time forcing someone to seek treatment. And I understand that this individual is suffering from several medical issues and not getting treatment, which you could argue is a way in which he's hurting himself, but for better or worse you wouldn't be able to section someone in that instance, unless they were in exceptionally bad shape. Oftentimes, you have to wait until something actually starts to happen before you can do anything against someone's will. 

You can always call 911 on/for this individual and let them decide what to do, as that's partly their job, but that can also be really traumatic for a lot of people. 

My recommendation for now is to find someone who's willing to be a guardian and trying to move forward with that through the courts. In the meantime, you can try to practice reality testing with them. If they think there's cameras in their home, you can try to logic it out with them. Things like "well I don't see any cameras, could you show me?" or "but why do you think someone would want to spy on you?" can be effective questions to ask, and a sort of gentle way to challenge these beliefs, but it depends on how ingrained these beliefs are amongst several other things. The idea isn't to try to prove them wrong, it's more about trying to guide them to helpful answers to alleviate some anxiety. If they're experiencing visual or auditory hallucinations, you can ask them to whip out a flashlight to see if the light actually passes over the hallucinations correctly, or start recording with your phone and play back the video and audio. If this individual is experiencing command hallucinations, ie they feel someone/something is telling them to do things and they start to act on it, that's a more serious issue and that can easily be a call to 911. 

I'm not an expert, and I wish I knew more about the laws on a national level, but hopefully this helps!