If you really think the idea won't work, ask him questions you think are critical for the idea to succeed. If he has all the answers--doesn't matter if they don't sound realistic to you--then leave him be. Most successful startup founders had plenty of people telling them the idea is stupid and will never work, but still succeeded. You may think it won't work based on your experience or knowledge but things change. For example a same idea can succeed or fail based on timing, execution, founder's network, etc.

Plus, failure is a part of the journey. Don't discourage someone from failing.