I'm sorry you and your son are going through this, I know how hard this can be!
My mum has delusions, and while I can't help tell you with what will make this easier, I can tell you what doesn't work.
Outright denying or trying to dissuade them from what they believe doesn't work. It's their reality, is like trying to persuade us that the sky isn't blue and the grass isn't green.
I'm no longer in contact with my mum (she was abusive. Her delusions had nothing to do with this decision), but when I was my therapist said instead to say things along the lines of "that must feel scary", etc, where you're not validating the delusion but you are their emotions around it.
As for paranoia, my therapist said that's how delusions are usually formed. The constant fear and worry had them hyperfocus on the thought, until they no longer can tell it's was originally just a fear they had and not their reality. With my mum, I noticed she would develop a delusion over the course of 1.5-2 weeks.
As for people who can help, I'm not sure there is actually all that much they can do. The reason mental illness exists is because there aren't any cures, and medication and therapy can only help so much.
My mum never took her medication enough to work and didn't go to therapy (so I don't have any experience in how it could help), but I know from my own experience that therapy doesn't work overnight, and that it doesn't work unless you put the work in and follow their advice. As such the reason it might not seem like it's working is because you're both expecting too much too soon, or he may not be following their advice.
The hardest lesson I've learnt in all of this, is you can't help someone who won't help themselves, and in my mum's case she refuses to help herself!
If you're struggling with this, it might be a good idea for you to talk to your GP to see if you qualify for some psychology sessions under Medicare. A psychologist would likely provide you great advice for how to provide better support and explain to you how things work from their perspective, while also teaching you ways to make sure you're helping yourself to stay emotionally strong and healthy!