[sadcodemonkey]

Look up the book titled "I am not sick I don't need help! How to help someone with mental illness accept treatment" by Xaviar Amador.

In a nutshell, the key is not to insist that your friend recognize that they're paranoid and suffering from delusions. You can't win that battle. Instead, get him to see that he can alleviate some of the immediate problems he's currently encountering (with work, family, doctors, you, etc) by getting an evaluation and medical treatment. In other words, to motivate him to get help, you might have to entertain some of his delusions to a certain degree. You have to play his mental game, until he (hopefully) gets treatment or medication and begins to see things more rationally.

If the two of you are not on speaking terms, find someone that he trusts and is willing to talk to, and get them to try this strategy.

This helped me when I was trying to convince a loved one with severe mental illness to make certain decisions about getting assistance and medical care that they were very resistant to.

Good luck. This is a very difficult and stressful thing to deal with, and as many others have advised, you should consider seeking support for your own mental health as well.