Tell him you found his pitch very interesting and persuasive, and that you'd like to buy life insurance for a 20 year period. Then, ponder for a little while; "Actually, it can't be having the contact that keeps them alive, can it? That's just a piece of paper. It must be that the sort of person who buy it are good at staying alive! And it looks like I'm one of them; this is excellent!

Then , you point out that as you're not going to die, you don't need life insurance, and say goodbye.

If you wanted to try to enlighten him, you might start by explicitly asking if he believed there was a causal link. But as the situation isn't really set up for honest truth-hunting, I wouldn't bother.